The treatment of benign prostatic hyperplasia (BPH) using Nd:YAG laser light has become an accepted alternative to TURP. However, there is no consensus to the dosimetry using the various laser devices. In our study, we evaluate the optical and thermal characteristics of 7 commercially available side firing laser probes. For the thermal analysis, an optical method was used based on 'Schlieren' techniques producing color images of the temperature distribution around the laser probe in water. Absolute temperatures were obtained after calibration measurements with thermocouples. Laser probes using metal mirrors for beam deflection heated up entirely. The local temperature rose up to 100 degrees centigrade, thus inducing vapor bubble formation that interfered with the emitted beam. Laser devices, using total internal reflection for deflection, showed far less heating primarily at the exit window, though Fresnel reflections and secondary beams indirectly heated up the (metal) housing of the tip. After clinical application, the absorption at the probe surface and hence temperature increased due to probe deterioration. Color Schlieren imaging is a powerful method for the thermal evaluation of laser devices. The thermal behavior of laser probes can be used as a guidance for the method of application and as an indication of the lifetime of the probes.