The inherited susceptibility to autoimmune Addison's disease was found to be strongly associated with human leukocyte antigens (HLA)-DR3 and DR4 alleles. In a study of 45 white patients from the United States with the disease, the relative risks (the number of times that an individual is at risk for Addison's disease if they had a marker, compared to those without such marker) were found to be 6.0, 4.6, and 26.5 for the DR3 allele, the DR4 allele, and for DR3/DR4 heterozygotes, respectively. Frequencies of DR2, DR5, and DR7 in the patients with Addison's disease were significantly decreased in comparison to 265 individuals in the control population. These HLA-DR frequencies in patients with Addison's disease were similar to those for 723 patients with insulin-dependent diabetes (IDD). However, the above HLA-DR associations persisted even when only data from the 37 patients with Addison's disease who did not have IDD were considered. Adrenocortical autoantibodies in 23 patients with IDD who did not have Addison's disease were equally frequent among those with DR4 and DR3 alleles. In contrast, HLA-DR frequencies in 17 patients with type I autoimmune polyglandular syndrome (chronic mucocutaneous moniliasis, hypoparathyroidism, Addison's disease, etc.) were not different from control. We conclude that genetic susceptibility to autoimmune Addison's disease may involve the same HLA-associated genetic determinants as IDD, except when Addison's disease occurs as part of type I autoimmune polyglandular syndrome.