In 280 patients with IDDM, the positive family history of diabetes was 26.8% of IDDM probands. The prevalence of diabetes in relatives was 68% in first degree relatives, 28% in second degree relatives, and 4% in third degree relatives. HLA typing of 87 members in 13 pedigrees with IDDM was performed. These data support the hypothesis that IDDM is a multigenic hereditary disorder.