BACKGROUND Telomerase is an enzyme that adds repeated telomere sequences to the ends of chromosome arms. It helps maintain both the length of telomere and infinite cell proliferation. In recent years, telomerase activity has been considered an important characteristic that differentiates between normal and cancerous cells. Because the authors often encountered difficulties in distinguishing between benign and malignant adrenal tumors, they investigated whether the expression of telomerase activity could distinguish potentially malignant adrenal tumors. METHODS The authors examined telomerase activity in 48 samples of adrenal tumor tissue and 27 samples of adjacent normal adrenal tissue. All samples were obtained from 48 patients who underwent surgery at Hamamatsu University Hospital in Hamamatsu, Japan. Based on the clinical and postoperative pathologic examinations, 45 samples were diagnosed as benign and 3 were diagnosed as malignant. Telomerase activity was examined using a telomerase repeat amplification protocol (TRAP) assay. RESULTS Of the 48 adrenal tumor samples, 7 (14.6%) had telomerase activity. All adjacent normal adrenal tissues were negative for telomerase activity. Of the telomerase positive samples, two were clinically known adrenocortical carcinoma, and another was metastatic adrenal tumor from lung carcinoma. Four other telomerase positive samples were diagnosed as benign after clinical and initial pathologic examinations. However, two of the patients from whom these samples were taken developed metastatic lesions after adrenalectomy. CONCLUSIONS A telomerase assay of adrenal tumors may help predict their malignant potential.