diff --git a/packages/x-data-grid/src/hooks/features/filter/gridFilterState.ts b/packages/x-data-grid/src/hooks/features/filter/gridFilterState.ts
index df84b7bf4aa37..3e5aa75d6cbc6 100644
--- a/packages/x-data-grid/src/hooks/features/filter/gridFilterState.ts
+++ b/packages/x-data-grid/src/hooks/features/filter/gridFilterState.ts
@@ -24,6 +24,7 @@ export interface GridFilterState {
   /**
    * Amount of children that are passing the filters or have children that are passing the filter (does not count grand children).
    * If a row is not registered in this lookup, it is supposed to have no descendant passing the filters.
+   * If `GridDataSource` is being used to load the data, the value is `-1` if the children count is unknown.
    */
   filteredChildrenCountLookup: Record<GridRowId, number>;
   /**