To define the role of asymptomatic autoimmune thyroiditis in the cause of breast cancer, the presence of circulating thyroid autoantibodies was studied in two populations, one with a high risk of breast cancer (British women) and one with a low risk (Japanese women). Ostensibly healthy women and patients with breast cancer from both countries were studied. There was no difference in the incidence of thyroid autoantibodies between women with breast cancer and healthy women in either race. The incidence of thyroid autoantibodies in healthy British women, however, was two to three times that in healthy Japanese women. The incidence of reticulin antibodies, was considerably higher in both groups of Japanese women. No remarkable differences in the incidence of antinuclear, smooth-muscle, antimitochondrial, gastric parietal cell, or liver-kidney microsomal antibodies were found between women with breast cancer and healthy women or between the two races. Only the incidence of antinuclear antibodies was marginally higher in Japanese patients with advanced cancer. These results indicate that asymptomatic autoimmune thyroid disease is more prevalent among British than among Japanese women, but they fail to provide direct evidence that autoimmune thyroid disease is associated with breast cancer. Prospective studies of women with autoimmune thyroiditis and studies of young women from low-risk and high-risk populations are needed.