The presence, distribution, and intensity of glomerular C1q localization were evaluated by direct immunofluorescence microscopy in 800 renal biopsy specimens which were also studied by light and electron microscopy. Identified were 15 patients with extensive (mean: 3.6 + out of 4 +), predominantly mesangial, C1q localization along with C3 and immunoglobulins, but no evidence for systemic lupus erythematosus. Pathologically, this lesion most closely resembled lupus nephritis. Clinical and pathologic data from these 15 C1q nephropathy patients were compared to data from 30 lupus nephritis and 223 other proliferative glomerulonephritis patients, and the C1q nephropathy patients were found to be dissimilar to both groups. The 15 C1q nephropathy patients had an average age of 17.8 years, 8 males, 7 females, 9 Black, 100% had proteinuria (mean 7.5 g/d), 40% hematuria, 0% hypocomplementemia, and 0% antinuclear antibodies. By electron microscopy, 100% had mesangial dense deposits, 20% capillary wall dense deposits, and 0% endothelial tubuloreticular inclusions. Nine patients treated with steroids had no definite resolution of proteinuria. We proposed that C1q nephropathy is a distinct clinicopathologic entity, usually causing steroid-resistant nephrotic syndrome in older children and young adults.