Rare cases of immunoglobulin G (IgG)-dominant immune complex–mediated glomerulonephritis demonstrate immunoglobulin subclass restriction without light chain restriction. Some of these cases may represent proliferative glomerulonephritis with monotypic immunoglobulin deposits (PGNMID) in which monotypic immunoglobulin is obscured by coexisting polytypic immunoglobulin. However, rigorous demonstration of this possibility is lacking to date. Here, we describe a case of IgG3-restricted immune complex–mediated glomerulonephritis without light chain restriction that apparently “transformed” into IgG3κ-PGNMID in a subsequent biopsy.