diff --git a/internal/pkgsite/client.go b/internal/pkgsite/client.go
index e9d1987..7ef4d59 100644
--- a/internal/pkgsite/client.go
+++ b/internal/pkgsite/client.go
@@ -15,6 +15,7 @@
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -23,11 +24,8 @@
 )
 
 type Client struct {
-	url string
-	// Cache of module paths already seen.
-	seen map[string]bool
-	// Does seen contain all known modules?
-	cacheComplete bool
+	url   string
+	cache *cache
 }
 
 func Default() *Client {
@@ -36,17 +34,13 @@
 
 func New(url string) *Client {
 	return &Client{
-		url:           url,
-		seen:          make(map[string]bool),
-		cacheComplete: false,
+		url:   url,
+		cache: newCache(),
 	}
 }
 
 func (pc *Client) SetKnownModules(known []string) {
-	for _, km := range known {
-		pc.seen[km] = true
-	}
-	pc.cacheComplete = true
+	pc.cache.setKnownModules(known)
 }
 
 // Limit pkgsite requests to this many per second.
@@ -64,13 +58,11 @@
 // Known reports whether pkgsite knows that modulePath actually refers
 // to a module.
 func (pc *Client) Known(ctx context.Context, modulePath string) (bool, error) {
-	// If we've seen it before, no need to call.
-	if b, ok := pc.seen[modulePath]; ok {
-		return b, nil
+	found, ok := pc.cache.lookup(modulePath)
+	if ok {
+		return found, nil
 	}
-	if pc.cacheComplete {
-		return false, nil
-	}
+
 	// Pause to maintain a max QPS.
 	if err := pkgsiteRateLimiter.Wait(ctx); err != nil {
 		return false, err
@@ -92,7 +84,7 @@
 		return false, err
 	}
 	known := res.StatusCode == http.StatusOK
-	pc.seen[modulePath] = known
+	pc.cache.add(modulePath, known)
 	return known, nil
 }
 
@@ -115,7 +107,10 @@
 	return seen, nil
 }
 
-func (c *Client) writeKnown(w io.Writer) error {
+func (c *cache) writeKnown(w io.Writer) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
 	b, err := json.MarshalIndent(c.seen, "", "   ")
 	if err != nil {
 		return err
@@ -164,7 +159,7 @@
 	if useRealPkgsite {
 		c := Default()
 		t.Cleanup(func() {
-			err := c.writeKnown(rw)
+			err := c.cache.writeKnown(rw)
 			if err != nil {
 				t.Error(err)
 			}
@@ -184,3 +179,54 @@
 	t.Cleanup(s.Close)
 	return New(s.URL), nil
 }
+
+type cache struct {
+	mu sync.Mutex
+	// Module paths already seen.
+	seen map[string]bool
+	// Does the cache contain all known modules?
+	complete bool
+}
+
+func newCache() *cache {
+	return &cache{
+		seen:     make(map[string]bool),
+		complete: false,
+	}
+}
+
+func (c *cache) setKnownModules(known []string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	for _, km := range known {
+		c.seen[km] = true
+	}
+	c.complete = true
+}
+
+func (c *cache) lookup(modulePath string) (known bool, ok bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	// In the cache.
+	if known, ok := c.seen[modulePath]; ok {
+		return known, true
+	}
+
+	// Not in the cache, but the cache is complete, so this
+	// module is not known.
+	if c.complete {
+		return false, true
+	}
+
+	// We can't make a statement about this module.
+	return false, false
+}
+
+func (c *cache) add(modulePath string, known bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.seen[modulePath] = known
+}
diff --git a/internal/pkgsite/client_test.go b/internal/pkgsite/client_test.go
index 7f3d952..153c522 100644
--- a/internal/pkgsite/client_test.go
+++ b/internal/pkgsite/client_test.go
@@ -43,3 +43,38 @@
 		})
 	}
 }
+
+func TestKnownParallel(t *testing.T) {
+	ctx := context.Background()
+	cf, err := CacheFile(t)
+	if err != nil {
+		t.Fatal(err)
+	}
+	pc, err := TestClient(t, *usePkgsite, cf)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, test := range []struct {
+		name string
+		in   string
+		want bool
+	}{
+		{name: "valid", in: "golang.org/x/mod", want: true},
+		{name: "invalid", in: "github.com/something/something", want: false},
+	} {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := pc.Known(ctx, test.in)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if got != test.want {
+				t.Errorf("%s: got %t, want %t", test.in, got, test.want)
+			}
+		})
+	}
+}
diff --git a/internal/pkgsite/testdata/pkgsite/TestKnownParallel.json b/internal/pkgsite/testdata/pkgsite/TestKnownParallel.json
new file mode 100644
index 0000000..45abe60
--- /dev/null
+++ b/internal/pkgsite/testdata/pkgsite/TestKnownParallel.json
@@ -0,0 +1,4 @@
+{
+   "github.com/something/something": false,
+   "golang.org/x/mod": true
+}
\ No newline at end of file