We have compared the therapeutic effectiveness of a new UVB fluorescent sunlamp, the Philips TL-01 lamp, which emits a narrow peak around 311-312 nm, with the currently used Philips TL-12 lamp, in 10 patients with psoriasis. We also compared the tumour inducing capacity of the two lamps in hairless mice. The therapeutic effect of the TL-01 lamp was superior to that of the TL-12 lamp in nine of the 10 patients. In the mice, the median tumour induction time was significantly longer in animals exposed to the TL-01 lamp. Phototherapy with the new type of lamp requires a higher dose than phototherapy with the usual broadband UVB sources. In practice this means that more lamps are needed in the light cabinet. However, the new lamps appear to provide more effective and safer phototherapy for psoriasis.