Following transurethral resection of bladder cancer, the fate of patients is clearly related to the presence or absence of precancerous or malignant lesions in the remaining mucosa. We report on a new procedure for in situ diagnosis of these flat, hardly visible urothelial lesions. The method is based on tumor-selective accumulation of endogenous protoporphyrin IX following intravesical instillation of 5-aminolevulinic acid. On excitation with violet light even tiny papillary tumors, dysplastic lesions and carcinoma in situ are bright fluorescing red. In 15 patients 26 neoplastic lesions had been diagnosed only by protoporphyrin IX fluorescence. False-negative results have not yet been observed. In 84% of 285 evaluable specimens the fluorescence findings corresponded correctly with the microscopic findings. It is expected that photodynamic diagnosis will become a matter of routine in detection of bladder cancer.