Surgery material from 28 cases was studied. Thyroid adenoma differs from thyroid carcinoma by a lower level of expression of Ki-67 and bcl-2, while the mutated p53 expression is lacking. These indices may be used for early and differential diagnosis of thyroid carcinoma. Medullary carcinoma is a tumor with the highest malignant potential and p53 and bcl-2 may serve as markers of a higher degree of malignancy. Ki-67 may serve a marker of proliferative activity of tumors belonging to the same histological type. Thus, its high expression in follicular carcinoma is an index of a high proliferative activity of its cells, correlates with its rapid growth and should be taken into consideration in therapy and prognosis. Expression of bcl-2 clearly correlates with neuroendocrine differentiation of carcinoma and its highest expression was found in the medullary carcinoma as well as in the chromogranin positive cells of the papillary thyroid carcinoma. APUD amyloid deposits in medullary carcinoma and high levels of c-myc in adenomas indicate some genetic restructurizations in malignant and benign thyroid tumors.