BACKGROUND Telomerase activity (TA) indicates malignancy, but activated lymphocytes also express TA. Correlation between TA in thyroid tissues and fine-needle aspiration (FNA) samples and knowledge about TA in adjacent tissue are of importance. METHODS The telomeric repeat amplification protocol assay followed by enzyme-linked immunosorbent assay detection was performed on 78 thyroid cases including 53 suspected malignancies, preoperative and perioperative FNA specimens, and adjacent tissue. RESULTS Benign lesions in cancer-suspected cases were TA negative. Eight of 13 papillary (62%) and 4 of 5 follicular (80%) tumors were TA positive (TA+). Lower TA was observed in conventional papillary cancer than in follicular, tall cell variant of papillary and anaplastic cancers. Adjacent tissues with lymphocyte infiltration were TA+ in 9 of 17 cases (53%). Nine of 65 adjacent tissues (14%) were TA+. Three of 6 preoperative and 9 of 11 perioperative FNA samples from malignant tumors corresponded to the tissue TA. CONCLUSIONS; High TA may reflect more severe thyroid cancer. Telomerase activity in FNA biopsies does not add reliable diagnostic information, and presence of lymphocytes can give false-positive results.