Interest in medical procedures such as Photodynamic Therapy (PDT), Interstitial Laser Hyperthermia (ILH), and Interstitial Laser Photocoagulation (ILP) is on the rise. Other medical procedures such as Cardiovascular-PDT (CV-PDT) and Laser Tissue Welding (LTW) are in their infancy. The one unifying parameter among these distinctly different disciplines is the use of a light source and a delivery system. Light sources are abundant and proliferating. Delivery systems are just evolving from second generation devices to more functional and user friendly third -generation devices. One such third generation device is the Flex Cylinder Diffuser (FCD) designed for use in interstitial PDT for the treatment of cancer. As an improvement to the FCD, a Guidewire Compatible Cylindrical Light Diffusing catheter has been designed. The guidewire compatible device potentially offers many advantages over second generation delivery systems such as: 1) safe delivery of the device to the target tissue; 2) a separate lumen to supply vital fluids to tissues obstructed by the device; 3) longitudinal and radial light uniformity for even treatment and simplified dosimetry; 4) higher power handling capabilities; 5) input mode independence; and 6) extreme flexibility. It is the goal of this project to develop this patented light diffusing catheter into a product useful for the above cited medical procedures. To that end the 1.0 cm long, 4.0 French diffuser will be capable of handling 4.0 Watts of input power at 660 nm wavelength with a longitudinal and radial light uniformity of +/- 33%. The methods of optimization and testing are detailed in the proposal along with the animal testing of the catheter in both rabbits and dogs.