The authors report the case of a 38 year old man with horseshoe kidney who developed a severe nephroso-nephritis syndrome, caused by cryoglobulinemic membranoproliferative glomerulo-nephritis. A combination of steroid and cyclophosphamide treatment resulted in partial improvement, but was discontinued after 12 weeks due to adverse reactions, with a consequent early relapse. The 4 week course of cyclosporine monotherapy proved ineffective and signs of cryoglobulinemia appeared. The elevation of transaminase, manifested during the immunosuppressive therapy demonstrated the presence of underlying chronic C hepatitis. In the light of the liver biopsy result, interferon treatment was commenced at a dose of 3 million unit thrice weekly. After 4 months of interferon treatment the persistent nephrotic range proteinuria decreased to below 0.5 g/day. Four months later clinical signs of cryoglobulinemia disappeared, and after the 10th month of interferon treatment no cryoglobulin could be detected in the patient's sera. After one year, the interferon treatment was discontinued following a negative PCR result for HCV. However, one month later the proteinuria increased and the quantitative hepatitis C virus nucleic acid test in sera became positive again. Our case demonstrates that interferon therapy may be effective in the treatment of cryoglobulinemic glomerulonephritis responding poorly to the immunosuppressive therapy, though larger doses or longer periods of treatment may be required to prevent relapses.