diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 1a5b4b675..030982d86 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -648,6 +648,10 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
  *
  * \return         0 if successful,
  *                 MBEDTLS_ERR_MPI_ALLOC_FAILED if memory allocation failed
+ *
+ * \note           The bytes obtained from the PRNG are interpreted
+ *                 as a big-endian representation of an MPI; this can
+ *                 be relevant in applications like deterministic ECDSA.
  */
 int mbedtls_mpi_fill_random( mbedtls_mpi *X, size_t size,
                      int (*f_rng)(void *, unsigned char *, size_t),