A 17-year-old male with solar urticaria is described. The action spectrum ranged from 400 to 520 nm. Wheals induced by visible light were inhibited by simultaneous or subsequent irradiation of the skin with UVA radiation. UVA irradiation prior to exposure to eliciting wavelengths revealed no inhibitory effect, nor was an inhibitory effect found by pre- or postirradiation of test sites with visible light longer than 530 nm. In vitro activation of the patient's serum by exposure to visible light induced wheal formation at the injection site. The wheal formed by in vitro-activated serum was suppressed only when the serum was exposed to UVA after, but not before irradiation with wavelengths of the action spectrum. This suggests that the inactivation of a photoallergen rather than of its precursor is the mechanism by which UVA exerts an inhibitory effect.