Human immunodeficiency virus-associated nephropathy (HIVAN), characterized by heavy proteinuria, rapidly progressive renal failure, "collapsing" glomerulopathy, and tubulointerstitial abnormalities, is the most common finding in HIV-infected patients undergoing a renal biopsy and predominantly affects blacks. We describe the clinical features and renal pathologic findings of 12 intravenous drug users (IVDUs) coinfected with HIV and hepatitis C virus (HCV) who were selected for renal biopsy because they presented with features different from typical HIVAN, including hypertension, microscopic hematuria, and cryoglobulinemia. There were seven black and five Hispanic patients. Eleven patients had immune complex glomerulonephritis (ICGN); one had glomerulosclerosis with immune complex deposits. Ten individuals had evidence of past hepatitis B viral infection, but none had persistent hepatitis B surface antigenemia. No other underlying cause for immune complex glomerulonephritis was identified. Renal biopsy showed membranoproliferative glomerulonephritis in five patients, mesangial proliferative glomerulonephritis in five, membranous nephropathy in one, and "collapsing" glomerulopathy with immune complex deposits in one. Hepatitis C virus RNA was detected by reverse transcription-polymerase chain reaction (RT-PCR) in the renal tissue and/or serum of nine of the 11 patients tested, and also in the renal biopsy tissue of four of eight patients with clinical and pathologic features of typical HIVAN without immunofluorescence evidence of immune complex deposits. One patient presented with renal failure, five patients developed end-stage renal disease (ESRD) requiring hemodialysis (mean time, 6.5 months), and six had stable renal function after a mean follow-up of 29.1 months (range, 2 to 72 months). Liver function abnormalities were present in seven of the 12 individuals, including four of the six patients who developed renal failure. These findings indicate that in some patients coinfected with HIV and HCV, the development of ICGN may dominate the clinical course of the disease. The occurrence of ICGN among black patients at risk for HIVAN may be related to the relatively high prevalence of HCV infection among IVDUs in this group.