Background: Polymorphous light eruption is a common chronic idiopathic photodermatosis. The action spectrum and therapy are under debate. Objective: The aim of the study was to analyze the clinical aspects of this dermatosis, the photodiagnostic tests, and the results of therapy in an academic center. Methods: To obtain a reasonable follow-up period, we examined all available data of the patients who underwent diagnostic phototests in the period 1985 through 1991. Our procedure of phototesting included determination of minimal erythema doses, photoprovocation tests, and photopatch tests. The evaluation of the effect of the therapy was based on the patients’ experiences, time spent outdoors, and amount of sun exposure. Results: Our collection included data on 35 men and 75 women. The age at onset differed significantly between men and women (averages 46 and 28 years, respectively; P < .01). The minimal erythema doses for UVB were lowered in 43% of the men and in 4% of the women (P < .01); the minimal erythema doses for UVA were lowered in 37% of the men and in 11% of the women (P < .01). The photoprovocation tests showed a pathologic reaction to both UVB and UVA in 88% of the men and in 52% of the women (P < .01). In the remaining patients we found pathologic reactions to UVB alone (for men 9%, for women 24%; P > .05) or UVA alone (for men 3%, for women 24%; P < .01). The abnormal reactions to visible light were almost exclusively observed in those patients who reacted pathologically to both UVB and UVA (43% of the male patients, 11% of the female patients; P < .01). The photopatch tests showed a large number of positive test results, mainly to skin care products or sunscreens (75% of all patients). The 70 most sensitive patients (64%) were treated with prophylactic UVB therapy 2 or 3 times a week at home or initially in the outpatient department. This treatment was normally done from February to June, but in severe cases throughout the whole year. Conclusion: Phototests revealed abnormal reactions to UVB as well as UVA and to some extent also to visible light. Prophylactic UVB therapy is a successful treatment for polymorphous light eruption. (J Am Acad Dermatol 2000;42:199-207.)