In recent years, it has been proposed that genetic admixture may have played a role in the increased frequency of insulin-dependent diabetes mellitus (IDDM) in young U.S. blacks relative to African blacks. In support of this proposal, the similar associations of specific markers of the major histocompatibility complex (MHC) with IDDM in U.S. blacks with respect to U.S. whites have been cited. To determine whether racial admixture was a factor in the increased prevalence, we did three analyses of admixture. In the first we used nine genetic markers (ABO, Rh, Fy, Hp, Gc, Pl, OR, Tfr, and Gm) and determined that there was significantly greater than zero genetic contribution from whites in our sample of U.S. black IDDM patients (9.6 ± 2.3%, P < 0.01) when a sample of U.S. blacks without IDDM was used as one 'parental' population. In the next two analyses, we estimated the amounts of genetic contribution from whites in the U.S. blacks with and without IDDM using reported gene frequencies for West African blacks for four genetic markers (ABO, Rh, Fy, and Hp). The estimate of admixture (21.4 ± 2.8%) for the black IDDM sample was greater than that for the U.S. black controls (17.9 ± 2.3%), although the difference was not significant. Our estimate of genetic contribution from whites, 21.4% for black IDDM patients, supports the assumptions of 20% admixture which MacDonald and Rotter and Hodge used to test their respective models for the inheritance of IDDM. These results support the hypothesis that admixture with the white population is, in part, responsible for the increase in prevalence of IDDM seen in U.S. blacks.