Photodynamic therapy (PDT) has recently attracted much attention, especially among urologists, because it appears to be a selective form of cancer treatment which causes minimal damage to normal surrounding tissues. In this study we made use of a new class of photosensitizers for the laser-induced fluorescence diagnosis (LIFD) and photodynamic therapy of human renal cell carcinoma xenotransplanted into nude mice. The purpose of this study was to evaluate the recently developed photosensitizing drug THOPP-MPEG for its efficacy as photosensitizer for LIFD and PDT of renal cell carcinoma. THOPP-MPEG was injected intraperitoneally (0.5 micrograms/g body weight) into the mice 6-8 days after tumor transplantation. On the 18th day after transplantation, the tumors reached a diameter of 3-4 mm. Seven days after administration of the drug the tumor-bearing kidney was irradiated percutaneously with a total light dose of 2 x 60 J/cm2 and a power density in the irradiated area of less than 150 mW/cm2. A continuous-beam argon-pumped dye laser (656 nm) was used. After excitation with laser light (488-514 nm), the vital tumor clusters and the surrounding tissues invaded with tumor cells showed intense red coloration by laser-induced fluorescence. Subsequent to the light exposure (656 nm), a heavy tumor necrosis of up to 3-5 mm resulted. No THOPP-MPEG phototoxicity in normal surrounding tissue at a dose of up to 100 mg/kg body weight was seen. We believe the future role of PDT in the management of tumors of the kidney to be adjuvant within the concept of conservative kidney-preserving surgery.