diff --git a/include/polarssl/ecdsa.h b/include/polarssl/ecdsa.h
index 2de3b68..4a29ac6 100644
--- a/include/polarssl/ecdsa.h
+++ b/include/polarssl/ecdsa.h
@@ -88,6 +88,7 @@
 /**
  * \brief           Compute ECDSA signature and write it to buffer,
  *                  serialized as defined in RFC 4492 page 20.
+ *                  (Not thread-safe to use same context in multiple threads)
  *
  * \param ctx       ECDSA context
  * \param hash      Message hash