With only a few exceptions, the ribonucleoprotein telomerase has been found in malignant, but not in benign tissues. Telomerase is thus a potentially new diagnostic marker. Carcinoma of the urinary bladder is the most frequent malignant tumor of the urinary tract and, after prostatic carcinoma, the second most common malignancy of the genitourinary system. In order to evaluate the diagnostic capabilities of telomerase in bladder carcinomas, four cell lines derived from human urothelial carcinomas of the bladder, 75 tissue samples from bladder carcinomas, eight tissue samples of normal bladder urothelium, 40 bladder washings and 30 urine samples were examined for telomerase activity. The four cell lines derived from urothelial carcinoma of the bladder (F975, 582, SCaBER, UM-UC-3) all exhibited high telomerase activity and were thus used as positive controls. Telomerase activity was found in nearly all (96%) tissue samples obtained from histologically confirmed urothelial carcinoma of the bladder. None of the normal tissue samples examined showed telomerase activity. Telomerase activity was similarly found in 73% of bladder washings in patients with histologically confirmed bladder carcinoma. There were no false positive results. The determination of telomerase activity in bladder washing samples thus represents a new diagnostic method for detection of tumor cells in rinsing media. Because of the early inactivation or degradation of telomerase there was no detection of the enzyme in native urine in the present study.