Subclinical hypothyroidism can be defined as an asymptomatic state in which a reduction in thyroid activity has been compensated by an increased TSH output to maintain a euthyroid state. We analysed clinical features, laboratory data, and pathologic findings in 39 cases of subclinical hypothyroidism who were diagnosed at the Dept. of Internal Medicine, Chungnam National University Hospital from Aug. 1984 to June, 1985. 1) The age distribution was from sixteen to sixty-nine and mean age was 34.8. Peak incidence was in the 4th decade and 3rd, 5th, 6th decade in order. 2) The sex distribution showed female preponderance with a ratio of 18.5 to 1. 3) The major presenting manifestations were nonspecific ones such as fatigue, indigestion, and anorexia. 4) Physical examination revealed diffuse goiter in 47.6%. Major abnormalities were no gross abnormality (30.9%), nodular goiter and facial edema. 5) There was no significant difference of the basal serum T3 and T4 concentrations between subclinical hypothyroidism and normal controls (p〉 0.05). 6) The basal serum TSH concentration of subclinical hypothyroidism (32.61±14.95 μU/ml) was significantly higher than that of normal controls (3.92±1.05 μU/ml) (p〈0.005). 7) Microsomal antibody was detected in 80.6% and thyroglobulin antibody was detected in 30%. 8) The pathologic findings in 26 cases revealed Hashimoto's thyroiditis in 76.9% (lymphocytic type, 34.6%; oxyphilic type, 26.9%; fibrotic type, 15.4%). The others were adenomatous goiter (15.4%), adenomatous carcinoma (3.8%) and subacute thyroiditis (3.8%).