Background: Phototoxicity is the most significant short-term adverse effect of PUVA therapy. Objective: We attempted to determine the incidence and possible causes of phototoxicity of sufficient degree to cause interruption of treatment. Methods: A retrospective study was conducted of 16,506 PUVA treatments given to 414 patients in two treatment centers. Results: Phototoxicity occurred in 10.9% of patients and was an adverse effect in 0.3% of treatments. Problems with the treatment protocol were the main cause. Conclusion: Phototoxicity is a common adverse effect, and patients should be warned of this potential occurrence. Awareness of the causes may help to reduce the incidence of this problem.