diff --git a/Tr/TrackMonitors/src/TrackVertexMonitor.cpp b/Tr/TrackMonitors/src/TrackVertexMonitor.cpp
index b6704591c69a46363f8d6b241e1c39d4a2dd7d3a..985be3d208ffac4d695950d8bcf534a430d9fc72 100644
--- a/Tr/TrackMonitors/src/TrackVertexMonitor.cpp
+++ b/Tr/TrackMonitors/src/TrackVertexMonitor.cpp
@@ -119,6 +119,8 @@ private:
       this, "PV left chisquare per dof", "PV left chisquare per dof", { 50, 0, 10 } };
   mutable Gaudi::Accumulators::Histogram<1> m_pvRightChisquarePerDof{
       this, "PV right chisquare per dof", "PV right chisquare per dof", { 50, 0, 10 } };
+  mutable Gaudi::Accumulators::Histogram<1> m_pvLeftRightDeltaNTracks{
+      this, "PV left-right nTracks", "PV left-right nTracks", { 100, -2, 2 } };
   mutable Gaudi::Accumulators::Histogram<1> m_pvForwardBackwardDeltaX{
       this, "PV forward-backward delta x", "PV forward-backward delta x", { 50, -0.1, 0.1 } };
   mutable Gaudi::Accumulators::Histogram<1> m_pvForwardBackwardDeltaY{
@@ -422,6 +424,9 @@ void TrackVertexMonitor::operator()( LHCb::RecVertex::Range const& pvcontainer,
         */
       }
       if ( leftvertex && rightvertex ) {
+        // draw the difference in number of tracks
+        ++m_pvLeftRightDeltaNTracks[( lefttracks.size() - righttracks.size() ) /
+                                    ( lefttracks.size() + righttracks.size() )];
         // draw the difference
         Gaudi::XYZVector dx = leftvertex->position() - rightvertex->position();