diff --git a/src/txorphanage.h b/src/txorphanage.h
index 207b79e0094..f3f73ce0f24 100644
--- a/src/txorphanage.h
+++ b/src/txorphanage.h
@@ -18,6 +18,7 @@
  * Since we cannot distinguish orphans from bad transactions with
  * non-existent inputs, we heavily limit the number of orphans
  * we keep and the duration we keep them for.
+ * Not thread-safe. Requires external synchronization.
  */
 class TxOrphanage {
 public: