diff --git a/toolkit/components/telemetry/Histograms.json b/toolkit/components/telemetry/Histograms.json
index fa5bde52d19a..9476719ce125 100644
--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -4817,6 +4817,14 @@
     "bug_numbers": [1305581],
     "description": "Whether or not a variable-length prefix set loaded from disk is corrupted (true = file corrupted)."
   },
+  "URLCLASSIFIER_VLPS_LONG_PREFIXES": {
+    "alert_emails": ["safebrowsing-telemetry@mozilla.org"],
+    "expires_in_version": "61",
+    "kind": "enumerated",
+    "n_values": 32,
+    "bug_numbers": [1322523],
+    "description": "Length of the first 20 long prefixes (> 4 bytes) received in a Safe Browsing V4 table during an update."
+  },
   "URLCLASSIFIER_LC_PREFIXES": {
     "record_in_processes": ["main", "content"],
     "alert_emails": ["safebrowsing-telemetry@mozilla.org"],
diff --git a/toolkit/components/url-classifier/HashStore.cpp b/toolkit/components/url-classifier/HashStore.cpp
index 69013cf31344..7d926c7cd8c4 100644
--- a/toolkit/components/url-classifier/HashStore.cpp
+++ b/toolkit/components/url-classifier/HashStore.cpp
@@ -42,6 +42,7 @@
 #include "zlib.h"
 #include "Classifier.h"
 #include "nsUrlClassifierDBService.h"
+#include "mozilla/Telemetry.h"
 
 // Main store for SafeBrowsing protocol data. We store
 // known add/sub chunks, prefixes and completions in memory
@@ -152,11 +153,21 @@ TableUpdateV2::NewSubComplete(uint32_t aAddChunk, const Completion& aHash, uint3
 void
 TableUpdateV4::NewPrefixes(int32_t aSize, std::string& aPrefixes)
 {
+  NS_ENSURE_TRUE_VOID(aSize >= 4 && aSize <= COMPLETE_SIZE);
   NS_ENSURE_TRUE_VOID(aPrefixes.size() % aSize == 0);
   NS_ENSURE_TRUE_VOID(!mPrefixesMap.Get(aSize));
 
-  if (LOG_ENABLED() && 4 == aSize) {
-    int numOfPrefixes = aPrefixes.size() / 4;
+  int numOfPrefixes = aPrefixes.size() / aSize;
+
+  if (aSize > 4) {
+    // TODO Bug 1364043 we may have a better API to record multiple samples into
+    // histograms with one call
+#ifdef NIGHTLY_BUILD
+    for (int i = 0; i < std::min(20, numOfPrefixes); i++) {
+      Telemetry::Accumulate(Telemetry::URLCLASSIFIER_VLPS_LONG_PREFIXES, aSize);
+    }
+#endif
+  } else if (LOG_ENABLED()) {
     uint32_t* p = (uint32_t*)aPrefixes.c_str();
 
     // Dump the first/last 10 fixed-length prefixes for debugging.