BACKGROUND: Photodynamic therapy (PDT) of superficial basal cell carcinomas using endogenous protoporphyrin IX generated by topically applied 5-aminolevulinic acid (5-ALA) is an innovative and clinically encouraging approach. OBJECTIVE: This paper reports on the first results of an ongoing clinical study on photodynamic therapy of 55 basal cell carcinomas in 19 patients. METHODS: Because of the chemical instability of 5-ALA, a novel nanocolloid lotion was developed. This formulation was applied to the lesions under occlusive dressing 6 h before irradiation with visible light from an unfiltered halogen lamp. Light-induced fluorescence was used to ascertain the superficial extension of the tumors. The response to the therapy was assessed by visual inspection and in 13 patients also by biopsies. RESULTS: After a single treatment, 47 (85%) basal cell carcinomas showed a complete response at a follow-up time of at least 6 months. For the remaining 8 (15%) lesions, a partial response was observed, indicated by a marked reduction of the tumor size. The treatment modality without local anesthetics was well tolerated by all patients, with only slight discomfort during light irradiation. CONCLUSION: Our results indicate that 5-ALA-PDT with our novel formulation, a nanocolloid lotion, is a highly effective and convenient therapy for the treatment of superficial basal cell carcinomas. Its use in the treatment of nodular tumors needs further investigations.