Background: Polymorphous light eruption (PLE) is a common form of photodermatosis. The pathogenesis of the disease and the mechanism of the hardening phenomenon, however, have not been clarified. Observation: We report a 62-year-old Japanese woman with PLE. Provocation and hardening induction testing revealed that hardening was induced at the sites irradiated by a sufficient dose of UV for reproduction of the lesion. Topical steroid could prevent a positive reaction but not the hardening induction. The expression of adhesion molecules and CD1a was decreased at the hardening site. Conclusion: From these results, the hardening phenomenon in our case might be due to immunosuppression by UV exposure supported by the down-regulation of cell adhesion molecule expression and partially due to the depletion of endogenous antigens which cause a delayed-type hypersensitivity reaction.