Chronic actinic dermatitis (CAD) has distinct clinical features different from polymoφhous light eruption (PLE). In order to clarify the difference between CAD and PLE, not only in clinical histories and findings but also in photobiological and histopathological reactions to phototests, we investigated 6 Japanese patients with CAD and performed provocative phototests which are our standardized methods in diagnosing PLE. On provocative phototests in CAD, pruritic papules were reproduced with smaller doses of UVB, at longer hours after irradiation (48-72 h) and they lasted for more days than in patients with PLE. Our study demonstrated that although milder cases of CAD and severe cases of PLE could not be distinguished clearly based on photobiological reactions alone, typical cases of CAD showed completely different provocative phototest results from those of PLE