Few occurrences of anti-glomerular basement membrane glomerulonephritis superimposed on membranous nephropathy have been reported; most reported cases began as membranous nephropathy. Little is known about the etiology, pathogenesis, or immunoglobulin G subclasses in glomerular basement membrane deposits in these combined cases. The authors report the case of an elderly patient with apparently simultaneous onset of both membranous nephropathy and anti-glomerular basement membrane glomerulonephritis who had an excellent outcome with only pulse corticosteroid therapy. Successive renal biopsy specimens showed occurrence and disappearance of immunoglobulin G4 deposits in the glomerular basement membrane. Immunoglobulin G subclass analysis was important in recognizing membranous nephropathy underlying the anti-glomerular basement membrane disease.