diff --git a/cartographer/mapping/internal/3d/local_trajectory_builder_3d.cc b/cartographer/mapping/internal/3d/local_trajectory_builder_3d.cc
index 8c979d1..314a8db 100644
--- a/cartographer/mapping/internal/3d/local_trajectory_builder_3d.cc
+++ b/cartographer/mapping/internal/3d/local_trajectory_builder_3d.cc
@@ -30,6 +30,8 @@
 namespace cartographer {
 namespace mapping {
 
+// TODO(spielawa): Adjust metrics for multi-trajectory. So far we assume a
+// single trajectory.
 static auto* kLocalSlamLatencyMetric = metrics::Gauge::Null();
 static auto* kLocalSlamVoxelFilterFraction = metrics::Gauge::Null();
 static auto* kLocalSlamScanMatcherFraction = metrics::Gauge::Null();