Background: Increased endogenous levels of protoporphyrin IX (PpIX) have been reported in the skin of patients with psoriasis. Activation of PpIX with blue light after exogenous topical application of its precursor, aminolevulinic acid, can improve psoriasis. Light activation of endogenous PpIX for the treatment of psoriasis has never been studied. Objective: The purpose of this study was to determine the efficacy of multiple exposures to blue light in patients with plaque psoriasis. Methods: A total of 17 patients with plaque psoriasis presenting increased endogenous level of PpIX were enrolled. Of the 2 identified plaques on each patient, 1 was exposed to 10 J/cm² of blue light from a fluorescent panel 3 times per week for 4 consecutive weeks. Plaque severity was assessed in a blinded fashion. PpIX levels were measured with in vivo fluorescence spectroscopy weekly. Results: A single exposure to blue light induced almost complete photobleaching of PpIX. There was no significant difference between the mean psoriasis severity score of the plaques before and after 12 exposures to blue light. Conclusion: Repeated exposure to blue light did not improve psoriasis. (J Am Acad Dermatol 2003;49:55-8.)