The treatment of Benign Prostatic Hyperplasia using the Nd:YAG laser is developing rapidly since the clinical introduction three years ago. In most cases a right angled fiber is used to deliver the laser light laterally towards the abundant prostatic tissue. The characteristics of these fibers differ, with regard to the angle at which the beam exits the fiber and the beam profile, resulting in a specific power density distribution on the tissue. During clinical use the characteristics of a fiber may change due to deterioration of the fiber tip. In this study the behavior of the various devices was monitored before and during clinical use with a special design power meter (Aquarius). The Aquarius measures the status of a right angled fiber in about fifteen seconds under clinical conditions, i.e., under water and at high input powers (40-80 Watt). In contrast with integrating sphere power meters specifically the primary beam is measured. The degree of deterioration of various fibers was quantified optically and thermally during clinical use. Devices using a metal mirror transmitted slightly less power than internal refraction based devices (80 versus 90 percent). The transmission of the various devices was not linear with the input power; at higher input powers (>30 Watts) vapor bubbles, that developed at the tip ofthe device, decreased the transmission. During clinical use there was a large variation in decrease of transmission with regard to the total amount of energy transmitted through the fiber. However, at the end ofa procedure the transmission had dropped to under 50 %. The Aquarius is a powerful tool for evaluation and comparison of different laser prostatectomy devices both for clinical and experimental studies.