To determine possible racial and age-related differences of focal autoimmune thyroiditis between white and black Americans, autopsy material from American subjects was evaluated for incidence and severity of chronic lymphocytic thyroiditis. The overall incidence of lymphocytic infiltration in thyroid sections from subjects aged more than 20 years was 41.4% in white females, 20% in white males, 17.4% in black females, and 8.5% in black males. An increase to 54.8% in incidence of thyroiditis from the first decade onward was noted in white females older than 80 years of age. Age did not cause a remarkable increase in incidence of thyroiditis in white males, in black females, or black males. The susceptibility to chronic thyroiditis in American white males and females was almost the same as that of British white males and females. The incidence of thyroiditis in black females and males was similar to that in Japanese females and males. These findings showed clear racial differences in susceptibility to chronic thyroiditis.