diff --git a/inc/SimInfo.hh b/inc/SimInfo.hh
index 81af4f1..dd2b276 100644
--- a/inc/SimInfo.hh
+++ b/inc/SimInfo.hh
@@ -29,6 +29,7 @@ namespace mu2e
     XYZVectorF endpos = XYZVectorF();  // end position of the SimParticle [mm, in detector coords]
     MCRelationship prirel = MCRelationship(); // relationship to the event primary particles
     MCRelationship trkrel = MCRelationship(); // relationship to the particle that created hits in the track
+    std::vector<int> ancestorSimIds; // full parent chain: [parentId, grandparentId, ..., rootId]
     void reset() { *this = SimInfo(); }
   };
 }
diff --git a/src/InfoMCStructHelper.cc b/src/InfoMCStructHelper.cc
index 61fb2e5..824d9fb 100644
--- a/src/InfoMCStructHelper.cc
+++ b/src/InfoMCStructHelper.cc
@@ -487,6 +487,12 @@ namespace mu2e {
         SimInfo siminfo;
         fillSimInfo(edep.sim(), siminfo);
         siminfo.index = csis.size();
+        // Walk the Geant4 parent chain and store all ancestor SimParticle IDs
+        auto currentPtr = edep.sim();
+        while (currentPtr->hasParent()) {
+          currentPtr = currentPtr->parent();
+          siminfo.ancestorSimIds.push_back(currentPtr->id().asInt());
+        }
         csis.push_back(siminfo);
       }
     }