Interleukin (IL)-17 is a key molecule for epithelial immunity and inflammation. Objectives: To quantify IL-17 expression in situ in a large panel of cutaneous diseases. 289 samples of the 30 most common cutaneous infectious, autoimmune, inflammatory and tumor diseases were stained for IL-17 immunohistochemically. IL-17 expression strongly varied between the diseases, but was conserved within each disease. The major cellular sources of IL-17 were T cells and granulocytes. Skin diseases caused by extracellular microbials were infiltrated by many IL-17+ cells, while intracellular infections were scarcely positive for IL-17. While autoimmune diseases were mostly accompanied by IL-17+ T cells, IL-17+ granulocytes were dominant in neutrophilic dermatoses. Cutaneous diseases show a characteristic pattern of IL-17+ cellular infiltrate. These patterns are relevant for the clinician, since therapeutic approaches targeting differentiation of Th17 cells as well as direct targeting of IL-17 are or will become available.