There is evidence of a genetic basis in some cases of idiopathic membranoproliferative glomerulonephritis (MPGN) types I and III, particularly those occurring in families. The clinical and morphological features and disease course in two siblings with MPGN are described. In the male sibling, both clinical and morphological features as well as serum complement profile suggested type I MPGN; electron microscopy appearance in the female sibling was consistent with type III MPGN. Both patients had treatment-resistant nephrotic syndrome which evolved into renal insufficiency in the girl. No hereditary complement deficiencies were found in siblings or their parents. Both children exhibited HLA-A24; -B27, w4; -DR11, 52; -DQ3 antigens. Between 1981 and 1996, 18 patients from eight families with unequivocal diagnosis of MPGN I or III had been described. The mode of inheritance appeared to be autosomal dominant or X-linked in four of these families. In 11 patients, including our 2, in whom HLA typing was performed, eight had the HLA-A2 antigen. Similarities and discrepancies regarding clinical and morphological features and outcomes were evident in these intrafamilial cases, suggesting either a similar genetic background or a multigenic origin of MPGN. The familial occurrence of the MPGN, highlighted by our report, supports the concept that genetically determined factors may be involved in the pathogenesis of the disease.