OBJECTIVE The crucial shortcoming of cystoscopy is that it does not measure the size of observed objects, and so we developed a new device that adds measurement capability to the cystoscope. METHODS The device consists of 5 arms linked to each other by freely bending joints. Before use 1 end of the arm is fixed to the examination table and the other end is linked to the eyepiece of the cystoscope. While linked to the arms, the cystoscope can move freely in any direction. Each joint carries an accurate sensor to measure its angle. The angle measurements are calculated collectively to obtain the 3-dimensional coordinates at the tip of the cystoscope. The tip of the cystoscope can be moved to the other side of the objects to be measured, which provides a pair of 3-dimensional coordinates, because the distance between them can be calculated. RESULTS We could calculate the distance between the bladder neck and each ureteral orifice, and the angle of the bladder neck formed with the left and right orifices in clinical cases. By continuous measurement of the location of the bladder neck and the ureteral orifice, it was clearly demonstrated that the distance between them increased as the bladder filled. CONCLUSIONS Our computer assisted, 3-dimensional mapping device can replace unreliable cystoscopic assessment with more reliable numerical values. New diagnostic criteria based on the exact numerical values can be established with the use of this device.