IgA nephropathy (Berger's disease) is an important cause of end-stage renal failure in persons of Asian and European descent. We performed C4 phenotyping on plasma from 123 patients with IgA nephropathy who resided in several different parts of the United States. All of these patients underwent diagnostic renal biopsy in adulthood. Six patients had a total deficiency for the C4A protein and all six had chronic renal insufficiency (serum creatinine concentration higher than 1.4 mg/dl at last follow-up). In contrast, 47% of the patients without C4A deficiency had chronic renal insufficiency (p = 0.001). The C4 gene defect was due to deletion of both C4A genes in only two individuals, whereas three patients were heterozygous for the C4A gene deletion. We speculate that the functional alteration of the complement system related to C4A deficiency might lead to expression of clinically severe disease in an individual with a genetic susceptibility to IgA nephropathy.