Clinicopathological studies of the histogenesis of the thyroid cancer on the emphasis of morphological features and growth pattern of the occult thyroid carcinoma measuring under 1.0 cm in diameter obtained at autopsy and surgery were performed. The occult thyroid carcinoma observed from the routine section of the thyroid gland obtained from 500 cases at autopsy and 259 cases at surgery were 21 cases (4.2%) and 10 cases (3.9%), respectively. Histological typing were 25 cases of papillary adenocarcinoma and 6 cases of follicular adenocarcinoma. Papillary adenocarcinoma cases were subdivided histologically into sclerosing, cystic and non-sclerosing types. The sclerosing type showed the proliferation of the fibrous connective tissue in the tumor and lymphogenous metastasis with intraglandular metastasis. One case of surgically obtained sclerosing type showed the recurrence and death. The cystic type were enclosed by the capsule and morphologically papillary cystadenocarcinoma. The non-sclerosing type did not show the fibrous connective tissue or capsular formation and smaller compared to the above 2 types. This type suggests the early stage of papillary adenocarcinoma histogenetically. Follicular adenocarcinoma cases were diagnosed according to the atypism of the tumor tissue and in 3 cases capsular formation was observed and in the other cases tumor tissue was directly attached to the thyroid tissue. Metastatic thyroid cancer was found in 12 cases (3.4%) among 349 autopsy cases of the malignant neoplasia. The mode of growth was either diffuse infiltration or solitary nodular formation in the thyroid tissue. Tumor like lesion (51 cases, 10.2%) was correspond to the so-called solid cell nest and suggested the squamas metaplasia according to the histological finding of the early histogenesis.