diff --git a/engine/src/main/java/com/google/android/fhir/sync/FhirSynchronizer.kt b/engine/src/main/java/com/google/android/fhir/sync/FhirSynchronizer.kt
index e1b300884d..a0020adb4a 100644
--- a/engine/src/main/java/com/google/android/fhir/sync/FhirSynchronizer.kt
+++ b/engine/src/main/java/com/google/android/fhir/sync/FhirSynchronizer.kt
@@ -25,6 +25,8 @@ import java.time.OffsetDateTime
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.SharedFlow
 import kotlinx.coroutines.flow.flow
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import org.hl7.fhir.r4.model.ResourceType
 
 enum class SyncOperation {
@@ -79,18 +81,20 @@ internal class FhirSynchronizer(
   }
 
   suspend fun synchronize(): SyncJobStatus {
-    setSyncState(SyncJobStatus.Started())
-
-    return listOf(download(), upload())
-      .filterIsInstance<SyncResult.Error>()
-      .flatMap { it.exceptions }
-      .let {
-        if (it.isEmpty()) {
-          setSyncState(SyncResult.Success())
-        } else {
-          setSyncState(SyncResult.Error(it))
+    mutex.withLock {
+      setSyncState(SyncJobStatus.Started())
+
+      return listOf(download(), upload())
+        .filterIsInstance<SyncResult.Error>()
+        .flatMap { it.exceptions }
+        .let {
+          if (it.isEmpty()) {
+            setSyncState(SyncResult.Success())
+          } else {
+            setSyncState(SyncResult.Error(it))
+          }
         }
-      }
+    }
   }
 
   private suspend fun download(): SyncResult {
@@ -141,4 +145,8 @@ internal class FhirSynchronizer(
       SyncResult.Error(exceptions)
     }
   }
+
+  companion object {
+    private val mutex = Mutex()
+  }
 }
diff --git a/engine/src/test/java/com/google/android/fhir/sync/FhirSynchronizerTest.kt b/engine/src/test/java/com/google/android/fhir/sync/FhirSynchronizerTest.kt
index e978562320..fe7b5f9116 100644
--- a/engine/src/test/java/com/google/android/fhir/sync/FhirSynchronizerTest.kt
+++ b/engine/src/test/java/com/google/android/fhir/sync/FhirSynchronizerTest.kt
@@ -24,11 +24,12 @@ import com.google.android.fhir.sync.upload.Uploader
 import com.google.android.fhir.testing.TestFhirEngineImpl
 import com.google.common.truth.Truth.assertThat
 import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.flowOf
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.test.UnconfinedTestDispatcher
 import kotlinx.coroutines.test.runTest
-import org.hl7.fhir.r4.model.Patient
 import org.hl7.fhir.r4.model.ResourceType
 import org.junit.Before
 import org.junit.Test
@@ -142,4 +143,65 @@ class FhirSynchronizerTest {
       assertThat(result).isInstanceOf(SyncJobStatus.Failed::class.java)
       assertThat(listOf(error)).isEqualTo((result as SyncJobStatus.Failed).exceptions)
     }
+
+  /**
+   * If you encounter flakiness in this test, consider increasing the delay time in the downloader
+   * object.
+   */
+  @Test
+  fun `synchronize multiple invocations should execute in order`() =
+    runTest(UnconfinedTestDispatcher()) {
+      `when`(downloader.download()).thenReturn(flowOf(DownloadState.Success(listOf(), 0, 0)))
+      `when`(uploader.upload(any()))
+        .thenReturn(
+          UploadSyncResult.Success(
+            listOf(),
+          ),
+        )
+      val fhirSynchronizerWithDelayInDownload =
+        FhirSynchronizer(
+          TestFhirEngineImpl,
+          UploadConfiguration(uploader),
+          DownloadConfiguration(
+            object : Downloader {
+              override suspend fun download(): Flow<DownloadState> {
+                delay(10)
+                return flowOf(DownloadState.Success(listOf(), 10, 10))
+              }
+            },
+            conflictResolver,
+          ),
+          fhirDataStore,
+        )
+      val emittedValues = mutableListOf<SyncJobStatus>()
+      val jobs =
+        listOf(fhirSynchronizerWithDelayInDownload, fhirSynchronizer).map {
+          backgroundScope.launch { it.syncState.collect { emittedValues.add(it) } }
+          /**
+           * Invoke synchronize() in separate coroutines. First invoke the synchronizer with delay
+           * in download. Then the [fhirSynchronizer]
+           */
+          backgroundScope.launch { it.synchronize() }
+        }
+
+      jobs.forEach { it.join() }
+
+      assertThat(emittedValues).hasSize(10)
+      assertThat(emittedValues[0]).isInstanceOf(SyncJobStatus.Started::class.java)
+      assertThat(emittedValues[1])
+        .isEqualTo(SyncJobStatus.InProgress(SyncOperation.DOWNLOAD, total = 10, completed = 10))
+      assertThat(emittedValues[2])
+        .isEqualTo(SyncJobStatus.InProgress(SyncOperation.UPLOAD, total = 1, completed = 0))
+      assertThat(emittedValues[3])
+        .isEqualTo(SyncJobStatus.InProgress(SyncOperation.UPLOAD, total = 1, completed = 1))
+      assertThat(emittedValues[4]).isInstanceOf(SyncJobStatus.Succeeded::class.java)
+      assertThat(emittedValues[5]).isInstanceOf(SyncJobStatus.Started::class.java)
+      assertThat(emittedValues[6])
+        .isEqualTo(SyncJobStatus.InProgress(SyncOperation.DOWNLOAD, total = 0, completed = 0))
+      assertThat(emittedValues[7])
+        .isEqualTo(SyncJobStatus.InProgress(SyncOperation.UPLOAD, total = 1, completed = 0))
+      assertThat(emittedValues[8])
+        .isEqualTo(SyncJobStatus.InProgress(SyncOperation.UPLOAD, total = 1, completed = 1))
+      assertThat(emittedValues[9]).isInstanceOf(SyncJobStatus.Succeeded::class.java)
+    }
 }