Polymorphous light eruption is a common seasonal photodermatosis with a typical history and clinical picture. In the interval, when no lesions are present, the diagnosis relies on artificial reproduction of polymorphous light eruption by phototesting. Photochemotherapy (psoralens with ultraviolet A [PUVA]) is currently an effective preventive treatment. One hundred sixty-seven patients with either a history of polymorphous light eruption or manifest disease entered our study. Of 142 patients tested, 49% developed typical lesions of polymorphous light eruption at the test sites. In 56% the action spectrum was found to be in the ultraviolet A range, in 17% in the ultraviolet B range, and in 26% in both ranges. A total of 122 patients received preventive treatment with PUVA. Of these, fifty-one returned for follow-up. Of the patients who were followed up, 64% reported total protection during outdoor activities in the summer, 26% reported partial protection, and 10% were not protected. Failure to improve was unrelated to the action spectrum. The action spectrum and the incidence of positive results on phototests in our patient population differed from those reported by others. It is possible that differences in the test protocols and in the light sources used may account for this discrepancy. There is clearly a need for a standardized test procedure. However, the majority of patients benefit from PUVA pretreatment regardless of their action spectrum.