Postmortem histological examination of the thyroid gland and measurement of serum antithyroid antibodies were performed in 70 patients without overt thyroid disease. Lymphocytic infiltration, antithyroglobulin hemagglutination antibody (TGHA), and antithyroid microsomal hemagglutination antibody (MCHA) were found in 12, 2, and 9 cases, respectively. The incidence of lymphocytic infiltration in females was three times that in males. Ten of the 12 cases with lymphocytic infiltration had positive antibodies (either TGHA or MCHA), while 10 of 11 patients with positive antibodies showed lymphocytic infiltration. Thus, the correlation between morphological and serological findings was highly significant at P less than 0.001. The incidence of a small thyroid gland of less than 15 g in weight was higher in patients with lymphocytic infiltration and/or positive antibodies than in patients with a normal thyroid gland. These data suggest that positive serum antithyroid antibodies in subjects without overt thyroid disease may indicate the existence of lymphocytic infiltration in the thyroid gland, that is presumably subclinical autoimmune thyroiditis.