On examination of the thyroid glands in 408 consecutive autopsy cases, 64 occult papillary carcinomas (OPC) were found in 46 cases (11.3%) whose age ranged from 16 to 82 years. Their incidence was 26 of 247 (10.5%) in male and 20/161 (12.4%) in female cases. No definite correlation was found between the incidence of carcinoma and sex or age. All 64 carcinomas were less than 7.7 mm in diameter. They exhibited nuclear characteristics of papillary carcinoma although their growth patterns were "follicular" in the tumors less than 1 mm, and "papillary" in 70% of the tumors more than 3 mm in diameter. The carcinomas were classified into three types according to the presence or absence of capsular formation and fibrosis: encapsulated tumors (ET), nonencapsulated sclerosing tumors (NEST), and nonsclerosing tumors (NST). The NEST (28 tumors) and NST (26 tumors) were found in persons of all ages, whereas ET (ten tumors) were found only in those of older than 53 years. The mean tumor diameters of the three types, 4.57 mm for ET, 2.10 mm for NEST, and 0.85 mm for NST, were significantly different. These findings suggested that papillary carcinomas arose as minute carcinomas showing follicular pattern at any age as NST and then changed to NEST and ET, and papillary pattern became prominent if the tumors continued to grow. Very minute carcinomas were detected in serial sections of two of 27 fibrosclerotic nodules. Occult papillary carcinomas were found at significantly high incidence in cases with adenomatous goiter.