diff --git a/include/mbedtls/arc4.h b/include/mbedtls/arc4.h
index 83a7461..c43f406 100644
--- a/include/mbedtls/arc4.h
+++ b/include/mbedtls/arc4.h
@@ -36,6 +36,7 @@
 
 #include <stddef.h>
 
+/* MBEDTLS_ERR_ARC4_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_ARC4_HW_ACCEL_FAILED                  -0x0019  /**< ARC4 hardware accelerator failed. */
 
 #ifdef __cplusplus