An unusual familial glomerular disease, characterized by the presence of diffuse round mesangial deposits of C3, is described in 2 siblings (1 male and 1 female) and their mother. The clinical picture in the 3 patients was a long-lasting proteinuria. An acute hemolytic uremic syndrome with malignant hypertension developed in the male at the age of 24 years, requiring bilateral nephrectomy. The glomerulonephritis recurred on a renal allograft. This disease is not HLA-linked and no characteristic abnormality of complement profile was seen in the 3 patients.