diff --git a/pc/sdp_offer_answer.cc b/pc/sdp_offer_answer.cc
index e4c1028e78..8a9849d815 100644
--- a/pc/sdp_offer_answer.cc
+++ b/pc/sdp_offer_answer.cc
@@ -120,8 +120,7 @@ const char kSimulcastDisabled[] = "WebRTC.PeerConnection.Simulcast.Disabled";
 static const int kRtcpCnameLength = 16;
 
 // The maximum length of the MID attribute.
-// TODO(bugs.webrtc.org/12517) - reduce to 16 again.
-static constexpr size_t kMidMaxSize = 32;
+static constexpr size_t kMidMaxSize = 16;
 
 const char kDefaultStreamId[] = "default";
 // NOTE: Duplicated in peer_connection.cc:
@@ -408,25 +407,21 @@ bool VerifyIceUfragPwdPresent(
 
 RTCError ValidateMids(const cricket::SessionDescription& description) {
   std::set<std::string> mids;
-  size_t max_length = 0;
   for (const cricket::ContentInfo& content : description.contents()) {
     if (content.name.empty()) {
       LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
                            "A media section is missing a MID attribute.");
     }
-    max_length = std::max(max_length, content.name.size());
     if (content.name.size() > kMidMaxSize) {
       LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
                            "The MID attribute exceeds the maximum supported "
-                           "length of 32 characters.");
+                           "length of 16 characters.");
     }
     if (!mids.insert(content.name).second) {
       LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
                            "Duplicate a=mid value '" + content.name + "'.");
     }
   }
-  RTC_HISTOGRAM_COUNTS_LINEAR("WebRTC.PeerConnection.Mid.Size", max_length, 0,
-                              31, 32);
   return RTCError::OK();
 }
 
diff --git a/pc/sdp_offer_answer_unittest.cc b/pc/sdp_offer_answer_unittest.cc
index f21d2b052e..ecac7f8de8 100644
--- a/pc/sdp_offer_answer_unittest.cc
+++ b/pc/sdp_offer_answer_unittest.cc
@@ -238,4 +238,30 @@ TEST_F(SdpOfferAnswerTest, BundleCodecCollisionInDifferentBundlesAllowed) {
              "WebRTC.PeerConnection.ValidBundledPayloadTypes", false));
 }
 
+TEST_F(SdpOfferAnswerTest, LargeMidsAreRejected) {
+  auto pc = CreatePeerConnection();
+  std::string sdp =
+      "v=0\r\n"
+      "o=- 0 3 IN IP4 127.0.0.1\r\n"
+      "s=-\r\n"
+      "t=0 0\r\n"
+      "a=fingerprint:sha-1 "
+      "4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB\r\n"
+      "a=setup:actpass\r\n"
+      "a=ice-ufrag:ETEn\r\n"
+      "a=ice-pwd:OtSK0WpNtpUjkY4+86js7Z/l\r\n"
+      "m=video 9 UDP/TLS/RTP/SAVPF 111\r\n"
+      "c=IN IP4 0.0.0.0\r\n"
+      "a=rtcp-mux\r\n"
+      "a=sendonly\r\n"
+      "a=rtpmap:111 VP8/90000\r\n"
+      "a=mid:01234567890123456\r\n";
+  auto desc = CreateSessionDescription(SdpType::kOffer, sdp);
+  ASSERT_NE(desc, nullptr);
+  RTCError error;
+  pc->SetRemoteDescription(std::move(desc), &error);
+  EXPECT_FALSE(error.ok());
+  EXPECT_EQ(error.type(), RTCErrorType::INVALID_PARAMETER);
+}
+
 }  // namespace webrtc