Copper vapor/dye systems are used for photodynamic therapy because they can produce several watts of light at 630 nm with high efficiency. In contrast to continuous wave argon/dye laser systems, the pulsed Cu/dye systems have a peak power 10,000 higher than the average power. Because no commercial spherical diffusing fibers guaranteed at these peak powers were available to us, we developed high power diffusers ourselves. The probes, with diameters up to 3.7 mm on 600 μm fibers, have an improved thermal behavior. The output is 50 - 60% of the laser beam input. Tested with input powers up to 6 W, no damage occurred for exposure times over 15 minutes. The isotropy of the probes varied between within 1% and 25% (sd) on average. We have successfully started photodynamic therapy in patients with carcinoma in situ in the bladder using these probes.