We describe a familial, relapsing type of hemolytic uremic syndrome (HUS) that has a possible genetic origin. Three members of a family, aged between 15 months and 34 years, evidenced HUS. Histological examination disclosed thrombotic microangiopathy. Recurrences, persistent hypocomplementemia, and the HLA haplotype A3,B7 were a common factor. We studied 17 members of this family and found a low C3 level from activation of the alternative pathway in the haplotype A3,B7 carriers. These three patients with HUS probably had a congenital and hereditary condition in which immunologic factors played a role in its pathogenesis.