Several studies have demonstrated that telomerase is activated and telomere length is altered in various types of tumors. In this study, we investigated telomerase activities and telomere length in 21 thyroid tumors. Telomerase activity was detected in 11 of 12 thyroid cancers and three of nine follicular adenomas. The mean telomere lengths in the cancers tissue and follicular adenomas were lower than in the respective background tissues, the differences being significant (P = 0.0055 and P < 0.006), respectively. Our findings suggest that change in telomerase activity and telomere length may be important for development of thyroid tumors. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.