We have developed a percutaneous whole layer core biopsy (WLCB) of bladder tumor and applied it successfully to stage invasive bladder tumor since April, 1985. We report here a modified method, trans-urethral WLCB (TU-WLCB) and present its clinical results. The patient was placed in the lithotomy position under caudal anesthesia, and a 20 F rigid nephroscope was introduced transurethrally. A newly developed 18-gauge (1.2 mm), 350 mm-long biopsy needle (Crown Core Cut Biopsy Needle) was advanced to the tumor through the nephroscope. A longitudinal transabdominal US was used to measure the distance between the needle tip and the outer layer of the bladder tumor. Because the needle advances 2.5 cm when fired by spring-loaded BIP Biopsy Gun (BIP, Germany), biopsy is performed by advancing the needle tip until the outer layer of tumor is slightly pierced under US guidance. Specimens of all 11 trans-urethral WLCB cases included the muscle layer and adipose tissue, and pathological staging were possible. TU-WLCB could be applied safely to tumors located at posterior wall and covered with the peritoneum. Serious complications were not observed so far. Thus, TU-WLCB may become a reliable technique to stage and evaluate neoadjuvant therapy for invasive bladder cancer.