Various side firing fibers have been developed in the past two years for Nd: YAG laser treatment of Benign Prostatic Hyperplasia (BPH). The method to deflect the beam laterally determines the power density at the urethral wall and consequent tissue effects. In this study the optical characteristics of eight different side firing fibers were evaluated by measuring transmission and beam profiles. A scanning device was developed which consisted of a sensor that was translated in two directions in front of the side firing fiber, while submerged in water. The transmission of the devices was measured by placing them in a transparent water filled tank in front of a power meter. The scans provided a three dimensional power density distribution of the fibers. The exit angle varied from 41 to 100 degrees, with respect to the fiber axis. The divergence of the beams was different in two directions, resulting in an elliptical spot at the urethral wall. The spot size ranged from 6.6 to 17.5 mm2 for a clinically relevant situation at 5 mm from the tip. The transmission of a new side firing fiber ranged from 43 to 83 per cent compared to a bare fiber. Due to the unique optical characteristics of each device, there is a large variation in the power density at the tissue and thus a specific dosimetry protocol for each fiber is required.