A group of Japanese were investigated for evidence of an association between Graves' disease and HL-A. Forty-four patients with the disease and 83 normal, unrelated random Japanese and Japanese-American controls were selected for study. The frequency of the W5 antigen among patients (57%) was significantly (P less than .0001) greater than among controls (20%). Of the 34 patients with abnormally elevated serum levels of anti-(thyroid) microsomal (anti-M) auto-antibodies, 56% had the W5 antigen. In contrast, of 48 control individuals tested for anti-M, only seven were seropositive and none (0%) of the seven had the W5 antigen. As expected the HL-A8 antigen was absent from this non-Caucasian population. These data demonstrate that the W5 antigen in Japanese, analogous to the HL-A8 antigen in Caucasians, is associated with Graves' disease but not with anti-M seropositivity in controls. The occurrence of different HL-A antigens in association with the same disease in different ethnic groups requires that the use of a major histocompatibility system antigen as a disease susceptibility marker must be confirmed for each ethnic group under study.