An epidemiological survey on the incidence of juvenile chronic lymphocytic thyroiditis was performed in 10,220 apparently healthy school children in Ishikawa district, Japan. The subject of present study included 6,244 school children (2,831 boys and 3,413 girls, ages 6-18 yrs.) in Kanazawa City and 3,976 children (2,055 boys and 1,921 girls, ages 6-18 yrs.) in Wajima City. The first group was selected as a representative of urban area and the second group as that of seaside area. Children who have goiter or firm thyroid were selected for testing antithyroglobulin and anti-microsomal antibodies in sera. Final diagnosis of chronic lymphocytic thyroiditis was made on histological specimen obtained by needle biopsy on the antibody positive subjects. The overall incidence of chronic lymphocytic thyroiditis in these children was 3.0 per 1,000, whereas the incidence in adolescent girls was as high as 8.2 per 1,000. There was a considerable sex difference in the prevalence, the ratio of female to male was 6.5:1, and the incidence increased with age. The incidence in seaside area was 5.3 per 1,000 that was significantly higher than in urban area, 1.4 per 1,000 (p less than 0.005). Histologically, 26 of 30 cases (87%) were classified as focal thyroiditis and 4 cases (13%) were diffuse thyroiditis. Serum T4-I and T3 values within normal range in all patients, but resting TSH was elevated in 1 of 23 cases and TSH response to TRH was exaggerated in 3 of 23 cases. Impaired organification of iodide was observed in 6 of 32 cases by iodide-perchlorate discharge test. The present study demonstrates that juvenile chronic lymphocytic thyroiditis is highly prevalent among apparently healthy school children and early recognition of the disease with preventive care for hypothyroidism in future should be stressed.