diff --git a/include/mbedtls/md4.h b/include/mbedtls/md4.h
index 1672e90..dc3c048 100644
--- a/include/mbedtls/md4.h
+++ b/include/mbedtls/md4.h
@@ -38,6 +38,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+/* MBEDTLS_ERR_MD4_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_MD4_HW_ACCEL_FAILED                   -0x002D  /**< MD4 hardware accelerator failed */
 
 #ifdef __cplusplus