diff --git a/mpt/disk.go b/mpt/disk.go
index efb022f..3c2c9ce 100644
--- a/mpt/disk.go
+++ b/mpt/disk.go
@@ -93,11 +93,12 @@ type diskTree struct {
 	pmem *pmem.Mem
 	mem  []byte // cache of pmem.Data()
 
-	file1  File
-	file2  File
-	leaf   File
-	closed bool
-	err    error // sticky error
+	file1            File
+	file2            File
+	leaf             File
+	persistedVersion int64
+	closed           bool
+	err              error // sticky error
 }
 
 // broken marks the tree broken with err as the reason.
@@ -211,6 +212,7 @@ func memOpen(file1, file2, disk File, op string) (_ Tree, err error) {
 	}
 
 	t.mem = t.pmem.Data()
+	t.persistedVersion = t.hdr().version()
 
 	return t, nil
 }
@@ -233,6 +235,7 @@ func (t *diskTree) Sync() error {
 	if err := t.pmem.Sync(); err != nil {
 		return t.broken(err)
 	}
+	t.persistedVersion = t.hdr().version()
 	return nil
 }
 
diff --git a/mpt/disk_test.go b/mpt/disk_test.go
index f101db6..dc7f2ca 100644
--- a/mpt/disk_test.go
+++ b/mpt/disk_test.go
@@ -171,6 +171,9 @@ func (tt *tester) reopen(minVer int64, minExact bool, format string, args ...any
 	if version < minVer || minExact != exact {
 		tt.t.Fatalf("reopen: %s: version = %d,%v, want ≥ %d,%v", kind, version, exact, minVer, minExact)
 	}
+	if tree.PersistedVersion() != version {
+		tt.t.Fatalf("reopen: %s: PersistedVersion = %d, want %d", kind, tree.PersistedVersion(), version)
+	}
 	if !exact {
 		f1.readOnly = false
 		f2.readOnly = false
@@ -437,3 +440,158 @@ func TestSetOverwriteDiskSize(t *testing.T) {
 		}
 	})
 }
+
+func TestPersistedVersion(t *testing.T) {
+	type step struct {
+		action        string // "check", "set", "snap", "sync", "reopen"
+		key           string
+		val           string
+		version       int64
+		wantVersion   int64
+		wantPersisted int64 // expected PersistedVersion on diskTree (on memTree, equals wantVersion)
+	}
+
+	tests := []struct {
+		name     string
+		diskOnly bool
+		steps    []step
+	}{
+		{
+			name: "initial_empty",
+			steps: []step{
+				{action: "check", wantVersion: 0, wantPersisted: 0},
+			},
+		},
+		{
+			name: "snap_without_sync",
+			steps: []step{
+				{action: "set", key: "k1", val: "v1", wantVersion: 0, wantPersisted: 0},
+				{action: "snap", version: 10, wantVersion: 10, wantPersisted: 0},
+			},
+		},
+		{
+			name: "snap_then_sync",
+			steps: []step{
+				{action: "set", key: "k1", val: "v1", wantVersion: 0, wantPersisted: 0},
+				{action: "snap", version: 10, wantVersion: 10, wantPersisted: 0},
+				{action: "sync", wantVersion: 10, wantPersisted: 10},
+			},
+		},
+		{
+			name:     "unflushed_crash_recovery",
+			diskOnly: true,
+			steps: []step{
+				{action: "set", key: "k1", val: "v1", wantVersion: 0, wantPersisted: 0},
+				{action: "snap", version: 10, wantVersion: 10, wantPersisted: 0},
+				{action: "sync", wantVersion: 10, wantPersisted: 10},
+				{action: "set", key: "k2", val: "v2", wantVersion: 10, wantPersisted: 10},
+				{action: "snap", version: 20, wantVersion: 20, wantPersisted: 10},
+				{action: "reopen", wantVersion: 10, wantPersisted: 10},
+			},
+		},
+		{
+			name: "negative_version_snap",
+			steps: []step{
+				{action: "set", key: "k1", val: "v1", wantVersion: 0, wantPersisted: 0},
+				{action: "snap", version: 10, wantVersion: 10, wantPersisted: 0},
+				{action: "sync", wantVersion: 10, wantPersisted: 10},
+				{action: "set", key: "k2", val: "v2", wantVersion: 10, wantPersisted: 10},
+				{action: "snap", version: -1, wantVersion: 10, wantPersisted: 10},
+				{action: "sync", wantVersion: 10, wantPersisted: 10},
+			},
+		},
+		{
+			name: "multiple_sync_cycles",
+			steps: []step{
+				{action: "set", key: "k1", val: "v1", wantVersion: 0, wantPersisted: 0},
+				{action: "snap", version: 10, wantVersion: 10, wantPersisted: 0},
+				{action: "sync", wantVersion: 10, wantPersisted: 10},
+				{action: "set", key: "k2", val: "v2", wantVersion: 10, wantPersisted: 10},
+				{action: "snap", version: 20, wantVersion: 20, wantPersisted: 10},
+				{action: "sync", wantVersion: 20, wantPersisted: 20},
+				{action: "set", key: "k3", val: "v3", wantVersion: 20, wantPersisted: 20},
+				{action: "snap", version: 30, wantVersion: 30, wantPersisted: 20},
+				{action: "sync", wantVersion: 30, wantPersisted: 30},
+			},
+		},
+	}
+
+	t.Run("diskTree", func(t *testing.T) {
+		for _, tc := range tests {
+			t.Run(tc.name, func(t *testing.T) {
+				f1 := new(memFile)
+				f2 := new(memFile)
+				f3 := new(memFile)
+
+				tree, err := New(f1, f2, f3)
+				check(t, err)
+				defer tree.Close()
+
+				for i, s := range tc.steps {
+					switch s.action {
+					case "set":
+						check(t, tree.Set(Key(s.key), Val(s.val)))
+					case "snap":
+						_, err := tree.Snap(s.version)
+						check(t, err)
+					case "sync":
+						check(t, tree.Sync())
+					case "reopen":
+						f1Copy := &memFile{data: slices.Clone(f1.data)}
+						f2Copy := &memFile{data: slices.Clone(f2.data)}
+						f3Copy := &memFile{data: slices.Clone(f3.data)}
+						check(t, tree.Close())
+						tree, err = New(f1Copy, f2Copy, f3Copy)
+						check(t, err)
+					case "check":
+						// Just assert versions.
+					default:
+						t.Fatalf("unknown action %q", s.action)
+					}
+
+					if v, _ := tree.Version(); v != s.wantVersion {
+						t.Fatalf("step %d (%s): Version() = %d, want %d", i, s.action, v, s.wantVersion)
+					}
+					if got := tree.PersistedVersion(); got != s.wantPersisted {
+						t.Fatalf("step %d (%s): PersistedVersion() = %d, want %d", i, s.action, got, s.wantPersisted)
+					}
+				}
+			})
+		}
+	})
+
+	t.Run("memTree", func(t *testing.T) {
+		for _, tc := range tests {
+			if tc.diskOnly {
+				continue
+			}
+			t.Run(tc.name, func(t *testing.T) {
+				tree := NewMemTree()
+				defer tree.Close()
+
+				for i, s := range tc.steps {
+					switch s.action {
+					case "set":
+						check(t, tree.Set(Key(s.key), Val(s.val)))
+					case "snap":
+						_, err := tree.Snap(s.version)
+						check(t, err)
+					case "sync":
+						check(t, tree.Sync())
+					case "check":
+						// Just assert versions.
+					default:
+						t.Fatalf("unknown action %q", s.action)
+					}
+
+					if v, _ := tree.Version(); v != s.wantVersion {
+						t.Fatalf("step %d (%s): Version() = %d, want %d", i, s.action, v, s.wantVersion)
+					}
+					if got := tree.PersistedVersion(); got != s.wantVersion {
+						t.Fatalf("step %d (%s): PersistedVersion() = %d, want %d", i, s.action, got, s.wantVersion)
+					}
+				}
+			})
+		}
+	})
+}
diff --git a/mpt/dmem.go b/mpt/dmem.go
index a6841fe..a82f667 100644
--- a/mpt/dmem.go
+++ b/mpt/dmem.go
@@ -126,6 +126,15 @@ func (t *diskTree) Version() (version int64, exact bool) {
 	return hdr.version(), hdr.exact()
 }
 
+// PersistedVersion returns the latest version number that has been
+// completely written and synced to disk.
+func (t *diskTree) PersistedVersion() int64 {
+	t.mmu.RLock()
+	defer t.mmu.RUnlock()
+
+	return t.persistedVersion
+}
+
 // Set sets the value associated with key to val.
 func (t *diskTree) Set(key Key, val Val) error {
 	t.mmu.RLock()
diff --git a/mpt/mem.go b/mpt/mem.go
index d4cbbc4..396795b 100644
--- a/mpt/mem.go
+++ b/mpt/mem.go
@@ -96,6 +96,12 @@ func (t *memTree) Version() (version int64, exact bool) {
 	return t.version, t.exact
 }
 
+// PersistedVersion returns the latest version number of the tree.
+// For an in-memory tree, this is the same as Version.
+func (t *memTree) PersistedVersion() int64 {
+	return t.version
+}
+
 // Snap returns a snapshot of t.
 func (t *memTree) Snap(version int64) (Snapshot, error) {
 	if t.err != nil {
diff --git a/mpt/tree.go b/mpt/tree.go
index 1cccd07..eed2ff8 100644
--- a/mpt/tree.go
+++ b/mpt/tree.go
@@ -236,6 +236,10 @@ type Tree interface {
 	// expected to replay all Set calls up to the next version.
 	Version() (version int64, exact bool)
 
+	// PersistedVersion returns the latest version number that has been
+	// completely written and synced to disk.
+	PersistedVersion() int64
+
 	// Close calls Sync and then closes the underlying files.
 	Close() error
 }