The incidence of chronic lymphocytic thyroiditis in autopsy material from Japanese and British subjects was evaluated. Lymphocytic infiltration in representative thyroid sections from 1826 Japanese cases collected from four different institutions was analysed. The overall incidence of lymphocytic infiltration was significantly higher in females (22.2 per cent) than in males (13.9 per cent). In females, the incidence reached 23.2 per cent in the fourth decade and showed no increase with age thereafter. The overall incidence of lymphocytic infiltration in thyroid sections from 810 British cases was 42.5 per cent in females and 19.4 per cent in males; an increase in the incidence of thyroiditis from the sixth decade onwards was noted in British females, the figure reaching 50.0 per cent in those aged over 70 years. These findings suggest possible racial differences in susceptibility to chronic thyroiditis. The disorder is not necessarily related to age, increasing severity of disease with age being found only in British females.