Photodynamic therapy consists of the combination of photosensitizers absorbing light in the visible spectral region and irradiation with light of corresponding wavelengths. We analysed its effects in comparison to PUVA treatment on cell lines MyLA and HuT78, established from patients with cutaneous T cell lymphomas. Proliferation was reduced to 50% by exposure to 7.5 J/cm2 UV-A (= ED50). This effect was increased more than 10-fold in the presence of 10 micrograms/ml 8-methoxypsoralen. The ED50 for photodynamic therapy using 630 nm light emitted by a dye laser and 10 micrograms/ml Photosan-3 was found to be about 1 J/cm2. In vivo fluorescence recordings during topical photodynamic therapy of mycosis fungoides lesions showed photo-bleaching and thus documented the triggering of photochemical reactions. Our observations document the capability of photodynamic therapy to inhibit proliferation of transformed T cells similar to PUVA, pointing out its potential in the treatment of cutaneous T cell lymphomas.