Polymorphic light eruption (PLE) is a photodermatosis with an extremely high prevalence, particularly among young women (up to 20%). The disease is characterized through itchy skin lesions on sun-exposed body sites occurring after sun exposure mostly in spring and early summer. Its etiopathogenesis is unknown but resistance to UV-induced immunosuppression with subsequent immune reactions against skin photoneoantigens has been suggested. Regulatory T cells (CD4+CD25+FoxP3+) (Tregs), a subset of T helper cells, are crucial for the induction of immunosuppression. We will test the hypothesis that PLE patients show pathogenic fluctuating Treg levels and function and related parameters over the seasons of the year, possibly being responsible for lack of immune modulation and autoimmunity in PLE. Natural or medical photohardening may normalize Treg deficiency in PLE and lead to clinical adaption in summer. Better insight into the pathogenesis of PLE may give clues to develop new therapeutic strategies.