Photodynamic diagnosis and therapy (PDT) with 5-aminolevulinic acid (ALA) is based on the tumor-selective accumulation of the photosensitizer protoporphyrin IX (PPIX). Endoscopic resection of a papillary noninvasive, low-grade bladder tumor after photodynamic diagnosis with ALA shows red fluorescence only in the tumor while the adjacent normal urothelium is negative (top: fluorescence histology, x100). After irradiation with light of appropriate wavelength and in the presence of oxygen, the photosensitizer will induce cellular damage by generating molecular oxygen. Endoscopic resection of a carcinoma in situ of the bladder after PDT with ALA reveals complete ablation of atypical cells (bottom: HE, x40). For details, see Wild et al. in this issue.