diff --git a/CHANGES b/CHANGES
index 3b0d0b54a041cd5ecd6c4422ba6849558b1a2a29..e1b33929d54cbc922298500572247a2e98791c55 100644
--- a/CHANGES
+++ b/CHANGES
@@ -3,6 +3,13 @@
  _______________
 
  Changes between 1.0.2 and 1.1.0  [xx XXX xxxx]
+  *) The return type of BIO_number_read() and BIO_number_written() as well as
+     the corresponding num_read and num_write members in the BIO structure has
+     changed from unsigned long to uint64_t. On platforms where an unsigned
+     long is 32 bits (e.g. Windows) these counters could overflow if >4Gb is
+     transferred.
+     [Matt Caswell]
+
   *) Given the pervasive nature of TLS extensions it is inadvisable to run
      OpenSSL without support for them. It also means that maintaining
      the OPENSSL_NO_TLSEXT option within the code is very invasive (and probably
diff --git a/crypto/bio/bio_lib.c b/crypto/bio/bio_lib.c
index cc859da74012e25adb220c9d75dfd4e1e181e29b..6ab471cbcb410ba15b300c3252fb97e8323f2627 100644
--- a/crypto/bio/bio_lib.c
+++ b/crypto/bio/bio_lib.c
@@ -209,7 +209,7 @@ int BIO_read(BIO *b, void *out, int outl)
     i = b->method->bread(b, out, outl);
 
     if (i > 0)
-        b->num_read += (unsigned long)i;
+        b->num_read += (uint64_t)i;
 
     if (cb != NULL)
         i = (int)cb(b, BIO_CB_READ | BIO_CB_RETURN, out, outl, 0L, (long)i);
@@ -242,7 +242,7 @@ int BIO_write(BIO *b, const void *in, int inl)
     i = b->method->bwrite(b, in, inl);
 
     if (i > 0)
-        b->num_write += (unsigned long)i;
+        b->num_write += (uint64_t)i;
 
     if (cb != NULL)
         i = (int)cb(b, BIO_CB_WRITE | BIO_CB_RETURN, in, inl, 0L, (long)i);
@@ -272,7 +272,7 @@ int BIO_puts(BIO *b, const char *in)
     i = b->method->bputs(b, in);
 
     if (i > 0)
-        b->num_write += (unsigned long)i;
+        b->num_write += (uint64_t)i;
 
     if (cb != NULL)
         i = (int)cb(b, BIO_CB_PUTS | BIO_CB_RETURN, in, 0, 0L, (long)i);
@@ -578,14 +578,14 @@ void *BIO_get_ex_data(BIO *bio, int idx)
     return (CRYPTO_get_ex_data(&(bio->ex_data), idx));
 }
 
-unsigned long BIO_number_read(BIO *bio)
+uint64_t BIO_number_read(BIO *bio)
 {
     if (bio)
         return bio->num_read;
     return 0;
 }
 
-unsigned long BIO_number_written(BIO *bio)
+uint64_t BIO_number_written(BIO *bio)
 {
     if (bio)
         return bio->num_write;
diff --git a/demos/easy_tls/easy-tls.c b/demos/easy_tls/easy-tls.c
index b9512909d14255f83009afa112d3e5f80abdb7b2..5b657803d072221a86963730fee6f9967112a95f 100644
--- a/demos/easy_tls/easy-tls.c
+++ b/demos/easy_tls/easy-tls.c
@@ -1020,7 +1020,7 @@ tls_proxy(int clear_fd, int tls_fd, int info_fd, SSL_CTX *ctx, int client_p)
         int clear_read_select = 0, clear_write_select = 0,
             tls_read_select = 0, tls_write_select = 0, progress = 0;
         int r;
-        unsigned long num_read = BIO_number_read(rbio),
+        uint64_t num_read = BIO_number_read(rbio),
             num_written = BIO_number_written(wbio);
 
         DEBUG_MSG2("loop iteration", ++tls_loop_count);
diff --git a/include/openssl/bio.h b/include/openssl/bio.h
index ab37c8781033edcbc3b52ec821d495d2a90bf959..e224dd94581973a363c374ae1f82f2c9abde343f 100644
--- a/include/openssl/bio.h
+++ b/include/openssl/bio.h
@@ -321,8 +321,8 @@ struct bio_st {
     struct bio_st *next_bio;    /* used by filter BIOs */
     struct bio_st *prev_bio;    /* used by filter BIOs */
     int references;
-    unsigned long num_read;
-    unsigned long num_write;
+    uint64_t num_read;
+    uint64_t num_write;
     CRYPTO_EX_DATA ex_data;
 };
 
@@ -618,8 +618,8 @@ int BIO_set_ex_data(BIO *bio, int idx, void *data);
 void *BIO_get_ex_data(BIO *bio, int idx);
 int BIO_get_ex_new_index(long argl, void *argp, CRYPTO_EX_new *new_func,
                          CRYPTO_EX_dup *dup_func, CRYPTO_EX_free *free_func);
-unsigned long BIO_number_read(BIO *bio);
-unsigned long BIO_number_written(BIO *bio);
+uint64_t BIO_number_read(BIO *bio);
+uint64_t BIO_number_written(BIO *bio);
 
 /* For BIO_f_asn1() */
 int BIO_asn1_set_prefix(BIO *b, asn1_ps_func *prefix,