Hepatitis C virus (HCV) infection is a major cause of chronic liver disease and is associated with a variety of extrahepatic manifestations, including cryoglobulinemia and glomerulonephritis. Epidemiologic evidence suggests that HCV infection may be a major risk factor for both cryoglobulinemic and type I membranoproliferative glomerulonephritis (MPGN). Clinical symptoms and laboratory data may or may not reflect the presence of chronic liver disease. Most patients have evidence of hypocomplementemia, circulating rheumatoid factors, and cryoglobulinemia. The pathogenesis of HCV-associated MPGN is probably a result of glomerular deposition of circulating HCV and anti-HCV antibodies. Treatment with interferon-cc has been shown to improve proteinuria, suppress viremia, and stabilize renal function. However, patients often relapse after therapy is stopped. The optimal therapy remains to be defined but may involve different dosage regimens of interferon-a or the combination of several antiviral agents.