diff --git a/crypto/bio/bio_test.c b/crypto/bio/bio_test.c
index cad4cf3..ee11acc 100644
--- a/crypto/bio/bio_test.c
+++ b/crypto/bio/bio_test.c
@@ -28,8 +28,8 @@
 #else
 #include <io.h>
 #pragma warning(push, 3)
-#include <WinSock2.h>
-#include <WS2tcpip.h>
+#include <winsock2.h>
+#include <ws2tcpip.h>
 #pragma warning(pop)
 #endif
 
diff --git a/crypto/bio/connect.c b/crypto/bio/connect.c
index 16cfd60..82f6a7e 100644
--- a/crypto/bio/connect.c
+++ b/crypto/bio/connect.c
@@ -68,8 +68,8 @@
 #include <unistd.h>
 #else
 #pragma warning(push, 3)
-#include <WinSock2.h>
-#include <WS2tcpip.h>
+#include <winsock2.h>
+#include <ws2tcpip.h>
 #pragma warning(pop)
 #endif
 
diff --git a/crypto/bio/fd.c b/crypto/bio/fd.c
index 6b70eac..0b5baca 100644
--- a/crypto/bio/fd.c
+++ b/crypto/bio/fd.c
@@ -64,7 +64,7 @@
 #else
 #include <io.h>
 #pragma warning(push, 3)
-#include <Windows.h>
+#include <windows.h>
 #pragma warning(pop)
 #endif
 
diff --git a/crypto/bio/socket.c b/crypto/bio/socket.c
index 75412cf..98f32a6 100644
--- a/crypto/bio/socket.c
+++ b/crypto/bio/socket.c
@@ -64,7 +64,7 @@
 #include <unistd.h>
 #else
 #pragma warning(push, 3)
-#include <WinSock2.h>
+#include <winsock2.h>
 #pragma warning(pop)
 
 #pragma comment(lib, "Ws2_32.lib")
diff --git a/crypto/bio/socket_helper.c b/crypto/bio/socket_helper.c
index 52e1606..197c737 100644
--- a/crypto/bio/socket_helper.c
+++ b/crypto/bio/socket_helper.c
@@ -26,8 +26,8 @@
 #include <unistd.h>
 #else
 #pragma warning(push, 3)
-#include <WinSock2.h>
-#include <WS2tcpip.h>
+#include <winsock2.h>
+#include <ws2tcpip.h>
 #pragma warning(pop)
 #endif
 
diff --git a/crypto/err/err.c b/crypto/err/err.c
index e89f95a..cbeb7c7 100644
--- a/crypto/err/err.c
+++ b/crypto/err/err.c
@@ -117,7 +117,7 @@
 
 #if defined(OPENSSL_WINDOWS)
 #pragma warning(push, 3)
-#include <Windows.h>
+#include <windows.h>
 #pragma warning(pop)
 #endif
 
diff --git a/crypto/mem.c b/crypto/mem.c
index a8a8c08..ce41440 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -66,7 +66,7 @@
 
 #if defined(OPENSSL_WINDOWS)
 #pragma warning(push, 3)
-#include <Windows.h>
+#include <windows.h>
 #pragma warning(pop)
 #else
 #include <strings.h>
diff --git a/crypto/rand/windows.c b/crypto/rand/windows.c
index 6f3f3a7..66d977f 100644
--- a/crypto/rand/windows.c
+++ b/crypto/rand/windows.c
@@ -21,7 +21,7 @@
 
 #pragma warning(push, 3)
 
-#include <Windows.h>
+#include <windows.h>
 
 /* #define needed to link in RtlGenRandom(), a.k.a. SystemFunction036.  See the
  * "Community Additions" comment on MSDN here:
diff --git a/crypto/thread.c b/crypto/thread.c
index fcf249e..5101617 100644
--- a/crypto/thread.c
+++ b/crypto/thread.c
@@ -61,7 +61,7 @@
 
 #if defined(OPENSSL_WINDOWS)
 #pragma warning(push, 3)
-#include <Windows.h>
+#include <windows.h>
 #pragma warning(pop)
 #endif
 
diff --git a/ssl/test/bssl_shim.cc b/ssl/test/bssl_shim.cc
index 468af1d..f69a654 100644
--- a/ssl/test/bssl_shim.cc
+++ b/ssl/test/bssl_shim.cc
@@ -24,8 +24,8 @@
 #else
 #include <io.h>
 #pragma warning(push, 3)
-#include <WinSock2.h>
-#include <WS2tcpip.h>
+#include <winsock2.h>
+#include <ws2tcpip.h>
 #pragma warning(pop)
 
 #pragma comment(lib, "Ws2_32.lib")
diff --git a/tool/speed.cc b/tool/speed.cc
index 6c8455c..d16b164 100644
--- a/tool/speed.cc
+++ b/tool/speed.cc
@@ -29,7 +29,7 @@
 
 #if defined(OPENSSL_WINDOWS)
 #pragma warning(push, 3)
-#include <Windows.h>
+#include <windows.h>
 #pragma warning(pop)
 #elif defined(OPENSSL_APPLE)
 #include <sys/time.h>
diff --git a/tool/transport_common.cc b/tool/transport_common.cc
index ddb41b6..9400fe6 100644
--- a/tool/transport_common.cc
+++ b/tool/transport_common.cc
@@ -34,8 +34,8 @@
 #define NOMINMAX
 #include <io.h>
 #pragma warning(push, 3)
-#include <WinSock2.h>
-#include <WS2tcpip.h>
+#include <winsock2.h>
+#include <ws2tcpip.h>
 #pragma warning(pop)
 
 typedef int ssize_t;