An association between Hashimoto's thyroiditis, the major histocompatibility loci and certain blood antigens has been sought. The diagnosis of thyroiditis was firmly established in 13 members of 4 families and in an additional 7 there was suggestive evidence of the disorder. Our investigation of HL-A antigens in these patients and other family members failed to reveal any consistent association with thyroiditis. There was also no linkage demonstrated between thyroiditis and blood or serum group antigens. Although linkage has been described between histocompatibility antigens and experimental thyroiditis in the mouse, such a relationship is lacking in Hashimoto's thyroiditis.