The frequency, distribution, and intensity of C1q localization were evaluated in 800 renal biopsy specimens, and these observations were correlated with light, immunofluorescence, and electron microscopy findings. Intense C1q immunostaining was most frequent in proliferative and membranous lupus glomerulonephritis and in a recently described form of proliferative glomerulonephritis designated "C1q nephropathy." Moderate intensity C1q immunostaining was observed in most cases of type I but not type II, membranoproliferative glomerulonephritis. Unlike lupus membranous glomerulopathy, non-lupus membranous glomerulopathy usually did not have extensive C1q localization. C1q was scanty or absent in IgA nephropathy and antiglomerular basement membrane antibody mediated glomerulonephritis. C1q, along with IgM and C3, was often present at sites of glomerular sclerosis, especially in focal segmental glomerulosclerosis. Extraglomerular C1q was most frequent and most intense in cases of lupus nephritis having extraglomerular immune deposits. The presence or absence and intensity of C1q immunostaining were shown to be useful in the differential diagnosis of some glomerulopathies.