Eight patients belonging to 3 unrelated families had biopsy-proven IgM mesangial nephropathy. In the first family, the mother and the 2 daughters were affected; in the second, the mother and the son; in the third, 2 sisters and the brother. Two additional sisters of the third family showed a clinical picture consistent with chronic glomerulonephritis. The clinical picture was that of hematuria and/or proteinuria. No patients had nephrotic syndrome. Genealogic investigation enabled us to discover 2 additional affected members in the kindred of the first family (1 with IgA nephropathy, 1 with clinical glomerulonephritis) and 3 other affected members in the pedigree of the third family (1 with IgA nephropathy, 1 with sclerosing glomerulonephritis, 1 with clinical glomerulonephritis). Immunogenetic studies showed the recurrence of an extended haplotype bearing DR beta 11-DQ beta 3B-DQ alpha 2-C4A3-C4B1-BfS in 9 of 10 affected members. Our data suggest that genetic factors may be involved in the mechanism of the disease and support the hypothesis that IgM nephropathy is a distinct disease entity.