PI6INK4A (P16) protein expression was analyzed immunohistochemically in archival material derived from 12 cases of follicular thyroid carcinoma, 57 cases of follicular adenoma and 17 cases of nodular goiter. Among follicular carcinomas, 11 out of 12 examined cases (91.7%) were positive for P161NK4A protein. Among follicular adenomas the percentage of immunopositivity was 76.5% (45/57) and among nodular goiter cases it was 19.3% (13/17). Overexpression of P16INK4A protein was found in 66.7% (8/12) of follicular carcinomas and in 19.3% (11/57) of follicular adenomas; the values of this parameter were statistically significantly higher in the follicular carcinoma group (p < 0.005). No P16INK4A protein overexpression was noted in nodular goiter cells. High immunohistochemically-detected expression of P16INK4A protein in follicular thyroid carcinoma cells suggests that the altered expression pattern of P16INK4A protein may disturb the regulatory mechanisms of thyreocyte cell cycle and plays a significant role in the formation of benign neoplasms and their malignant counterparts derived from follicular thyroid cell.