Telomerase activity is known to be absent from most normal and well-differentiated tissues, although being detectable in the vast majority of malignant tumors. An increasing number of reports demonstrate that telomerase may be activated in benign tumors, such as adenomas. We have investigated a series of normal and neoplastic thyroid tissues for the presence of telomerase activity. As expected, all normal thyroid tissues (n = 20) had no display of telomerase activity. Amongst cancers, the incidence of telomerase activity varied with the histological subtypes. Telomerase activity was present in only 3/15 cases (20%) of papillary carcinomas. Telomerase activity was more frequently detected in follicular (4/6) and in undifferentiated (2/3) carcinomas. Unexpectedly, one case (1/12) of adenoma contained high levels of telomerase activity. Taken together, these results indicate that telomerase may play some role in the pathogenesis of thyroid tumors, in particular in follicular and undifferentiated carcinomas that are known to have the most aggressive behavior.