diff --git a/src/database/sql/sql.go b/src/database/sql/sql.go
index adf9649..04986a2 100644
--- a/src/database/sql/sql.go
+++ b/src/database/sql/sql.go
@@ -1554,19 +1554,6 @@
 	tx.closemu.RLock()
 	defer tx.closemu.RUnlock()
 
-	// TODO(bradfitz): We could be more efficient here and either
-	// provide a method to take an existing Stmt (created on
-	// perhaps a different Conn), and re-create it on this Conn if
-	// necessary. Or, better: keep a map in DB of query string to
-	// Stmts, and have Stmt.Execute do the right thing and
-	// re-prepare if the Conn in use doesn't have that prepared
-	// statement. But we'll want to avoid caching the statement
-	// in the case where we only call conn.Prepare implicitly
-	// (such as in db.Exec or tx.Exec), but the caller package
-	// can't be holding a reference to the returned statement.
-	// Perhaps just looking at the reference count (by noting
-	// Stmt.Close) would be enough. We might also want a finalizer
-	// on Stmt to drop the reference count.
 	dc, err := tx.grabConn(ctx)
 	if err != nil {
 		return nil, err
@@ -1621,11 +1608,6 @@
 	tx.closemu.RLock()
 	defer tx.closemu.RUnlock()
 
-	// TODO(bradfitz): optimize this. Currently this re-prepares
-	// each time. This is fine for now to illustrate the API but
-	// we should really cache already-prepared statements
-	// per-Conn. See also the big comment in Tx.Prepare.
-
 	if tx.db != stmt.db {
 		return &Stmt{stickyErr: errors.New("sql: Tx.Stmt: statement from different database used")}
 	}
@@ -1634,9 +1616,45 @@
 		return &Stmt{stickyErr: err}
 	}
 	var si driver.Stmt
-	withLock(dc, func() {
-		si, err = ctxDriverPrepare(ctx, dc.ci, stmt.query)
-	})
+	var parentStmt *Stmt
+	stmt.mu.Lock()
+	if stmt.closed || stmt.tx != nil {
+		// If the statement has been closed or already belongs to a
+		// transaction, we can't reuse it in this connection.
+		// Since tx.StmtContext should never need to be called with a
+		// Stmt already belonging to tx, we ignore this edge case and
+		// re-prepare the statement in this case. No need to add
+		// code-complexity for this.
+		stmt.mu.Unlock()
+		withLock(dc, func() {
+			si, err = ctxDriverPrepare(ctx, dc.ci, stmt.query)
+		})
+		if err != nil {
+			return &Stmt{stickyErr: err}
+		}
+	} else {
+		stmt.removeClosedStmtLocked()
+		// See if the statement has already been prepared on this connection,
+		// and reuse it if possible.
+		for _, v := range stmt.css {
+			if v.dc == dc {
+				si = v.ds.si
+				break
+			}
+		}
+
+		stmt.mu.Unlock()
+
+		if si == nil {
+			cs, err := stmt.prepareOnConnLocked(ctx, dc)
+			if err != nil {
+				return &Stmt{stickyErr: err}
+			}
+			si = cs.si
+		}
+		parentStmt = stmt
+	}
+
 	txs := &Stmt{
 		db: tx.db,
 		tx: tx,
@@ -1644,8 +1662,11 @@
 			Locker: dc,
 			si:     si,
 		},
-		query:     stmt.query,
-		stickyErr: err,
+		parentStmt: parentStmt,
+		query:      stmt.query,
+	}
+	if parentStmt != nil {
+		tx.db.addDep(parentStmt, txs)
 	}
 	tx.stmts.Lock()
 	tx.stmts.v = append(tx.stmts.v, txs)
@@ -1769,13 +1790,21 @@
 	tx   *Tx
 	txds *driverStmt
 
+	// parentStmt is set when a transaction-specific statement
+	// is requested from an identical statement prepared on the same
+	// conn. parentStmt is used to track the dependency of this statement
+	// on its originating ("parent") statement so that parentStmt may
+	// be closed by the user without them having to know whether or not
+	// any transactions are still using it.
+	parentStmt *Stmt
+
 	mu     sync.Mutex // protects the rest of the fields
 	closed bool
 
 	// css is a list of underlying driver statement interfaces
 	// that are valid on particular connections. This is only
 	// used if tx == nil and one is found that has idle
-	// connections. If tx != nil, txsi is always used.
+	// connections. If tx != nil, txds is always used.
 	css []connStmt
 
 	// lastNumClosed is copied from db.numClosed when Stmt is created
@@ -1916,20 +1945,30 @@
 
 	// No luck; we need to prepare the statement on this connection
 	withLock(dc, func() {
-		ds, err = dc.prepareLocked(ctx, s.query)
+		ds, err = s.prepareOnConnLocked(ctx, dc)
 	})
 	if err != nil {
 		s.db.putConn(dc, err)
 		return nil, nil, nil, err
 	}
-	s.mu.Lock()
-	cs := connStmt{dc, ds}
-	s.css = append(s.css, cs)
-	s.mu.Unlock()
 
 	return dc, dc.releaseConn, ds, nil
 }
 
+// prepareOnConnLocked prepares the query in Stmt s on dc and adds it to the list of
+// open connStmt on the statement. It assumes the caller is holding the lock on dc.
+func (s *Stmt) prepareOnConnLocked(ctx context.Context, dc *driverConn) (*driverStmt, error) {
+	si, err := dc.prepareLocked(ctx, s.query)
+	if err != nil {
+		return nil, err
+	}
+	cs := connStmt{dc, si}
+	s.mu.Lock()
+	s.css = append(s.css, cs)
+	s.mu.Unlock()
+	return cs.ds, nil
+}
+
 // QueryContext executes a prepared query statement with the given arguments
 // and returns the query results as a *Rows.
 func (s *Stmt) QueryContext(ctx context.Context, args ...interface{}) (*Rows, error) {
@@ -2056,11 +2095,16 @@
 	s.closed = true
 	s.mu.Unlock()
 
-	if s.tx != nil {
-		return s.txds.Close()
+	if s.tx == nil {
+		return s.db.removeDep(s, s)
 	}
 
-	return s.db.removeDep(s, s)
+	if s.parentStmt != nil {
+		// If parentStmt is set, we must not close s.txds since it's stored
+		// in the css array of the parentStmt.
+		return s.db.removeDep(s.parentStmt, s)
+	}
+	return s.txds.Close()
 }
 
 func (s *Stmt) finalClose() error {
diff --git a/src/database/sql/sql_test.go b/src/database/sql/sql_test.go
index 8441aff..79732d4 100644
--- a/src/database/sql/sql_test.go
+++ b/src/database/sql/sql_test.go
@@ -1024,6 +1024,196 @@
 	}
 }
 
+func TestTxStmtPreparedOnce(t *testing.T) {
+	db := newTestDB(t, "")
+	defer closeDB(t, db)
+	exec(t, db, "CREATE|t1|name=string,age=int32")
+
+	prepares0 := numPrepares(t, db)
+
+	// db.Prepare increments numPrepares.
+	stmt, err := db.Prepare("INSERT|t1|name=?,age=?")
+	if err != nil {
+		t.Fatalf("Stmt, err = %v, %v", stmt, err)
+	}
+	defer stmt.Close()
+
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatalf("Begin = %v", err)
+	}
+
+	txs1 := tx.Stmt(stmt)
+	txs2 := tx.Stmt(stmt)
+
+	_, err = txs1.Exec("Go", 7)
+	if err != nil {
+		t.Fatalf("Exec = %v", err)
+	}
+	txs1.Close()
+
+	_, err = txs2.Exec("Gopher", 8)
+	if err != nil {
+		t.Fatalf("Exec = %v", err)
+	}
+	txs2.Close()
+
+	err = tx.Commit()
+	if err != nil {
+		t.Fatalf("Commit = %v", err)
+	}
+
+	if prepares := numPrepares(t, db) - prepares0; prepares != 1 {
+		t.Errorf("executed %d Prepare statements; want 1", prepares)
+	}
+}
+
+func TestTxStmtClosedRePrepares(t *testing.T) {
+	db := newTestDB(t, "")
+	defer closeDB(t, db)
+	exec(t, db, "CREATE|t1|name=string,age=int32")
+
+	prepares0 := numPrepares(t, db)
+
+	// db.Prepare increments numPrepares.
+	stmt, err := db.Prepare("INSERT|t1|name=?,age=?")
+	if err != nil {
+		t.Fatalf("Stmt, err = %v, %v", stmt, err)
+	}
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatalf("Begin = %v", err)
+	}
+	err = stmt.Close()
+	if err != nil {
+		t.Fatalf("stmt.Close() = %v", err)
+	}
+	// tx.Stmt increments numPrepares because stmt is closed.
+	txs := tx.Stmt(stmt)
+	if txs.stickyErr != nil {
+		t.Fatal(txs.stickyErr)
+	}
+	if txs.parentStmt != nil {
+		t.Fatal("expected nil parentStmt")
+	}
+	_, err = txs.Exec(`Eric`, 82)
+	if err != nil {
+		t.Fatalf("txs.Exec = %v", err)
+	}
+
+	err = txs.Close()
+	if err != nil {
+		t.Fatalf("txs.Close = %v", err)
+	}
+
+	tx.Rollback()
+
+	if prepares := numPrepares(t, db) - prepares0; prepares != 2 {
+		t.Errorf("executed %d Prepare statements; want 2", prepares)
+	}
+}
+
+func TestParentStmtOutlivesTxStmt(t *testing.T) {
+	db := newTestDB(t, "")
+	defer closeDB(t, db)
+	exec(t, db, "CREATE|t1|name=string,age=int32")
+
+	// Make sure everything happens on the same connection.
+	db.SetMaxOpenConns(1)
+
+	prepares0 := numPrepares(t, db)
+
+	// db.Prepare increments numPrepares.
+	stmt, err := db.Prepare("INSERT|t1|name=?,age=?")
+	if err != nil {
+		t.Fatalf("Stmt, err = %v, %v", stmt, err)
+	}
+	defer stmt.Close()
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatalf("Begin = %v", err)
+	}
+	txs := tx.Stmt(stmt)
+	if len(stmt.css) != 1 {
+		t.Fatalf("len(stmt.css) = %v; want 1", len(stmt.css))
+	}
+	err = txs.Close()
+	if err != nil {
+		t.Fatalf("txs.Close() = %v", err)
+	}
+	err = tx.Rollback()
+	if err != nil {
+		t.Fatalf("tx.Rollback() = %v", err)
+	}
+	// txs must not be valid.
+	_, err = txs.Exec("Suzan", 30)
+	if err == nil {
+		t.Fatalf("txs.Exec(), expected err")
+	}
+	// Stmt must still be valid.
+	_, err = stmt.Exec("Janina", 25)
+	if err != nil {
+		t.Fatalf("stmt.Exec() = %v", err)
+	}
+
+	if prepares := numPrepares(t, db) - prepares0; prepares != 1 {
+		t.Errorf("executed %d Prepare statements; want 1", prepares)
+	}
+}
+
+// Test that tx.Stmt called with a statment already
+// associated with tx as argument re-prepares the same
+// statement again.
+func TestTxStmtFromTxStmtRePrepares(t *testing.T) {
+	db := newTestDB(t, "")
+	defer closeDB(t, db)
+	exec(t, db, "CREATE|t1|name=string,age=int32")
+	prepares0 := numPrepares(t, db)
+	// db.Prepare increments numPrepares.
+	stmt, err := db.Prepare("INSERT|t1|name=?,age=?")
+	if err != nil {
+		t.Fatalf("Stmt, err = %v, %v", stmt, err)
+	}
+	defer stmt.Close()
+
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatalf("Begin = %v", err)
+	}
+	txs1 := tx.Stmt(stmt)
+
+	// tx.Stmt(txs1) increments numPrepares because txs1 already
+	// belongs to a transaction (albeit the same transaction).
+	txs2 := tx.Stmt(txs1)
+	if txs2.stickyErr != nil {
+		t.Fatal(txs2.stickyErr)
+	}
+	if txs2.parentStmt != nil {
+		t.Fatal("expected nil parentStmt")
+	}
+	_, err = txs2.Exec(`Eric`, 82)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = txs1.Close()
+	if err != nil {
+		t.Fatalf("txs1.Close = %v", err)
+	}
+	err = txs2.Close()
+	if err != nil {
+		t.Fatalf("txs1.Close = %v", err)
+	}
+	err = tx.Rollback()
+	if err != nil {
+		t.Fatalf("tx.Rollback = %v", err)
+	}
+
+	if prepares := numPrepares(t, db) - prepares0; prepares != 2 {
+		t.Errorf("executed %d Prepare statements; want 2", prepares)
+	}
+}
+
 // Issue: https://golang.org/issue/2784
 // This test didn't fail before because we got lucky with the fakedb driver.
 // It was failing, and now not, in github.com/bradfitz/go-sql-test