Background: Psoriasis is a common, chronic, inflammatory disease of multiple etiological factors, mainly immunological, affecting skin and joints. Many cytokines produced by T lymphocytes, especially T helper type, have been implicated in its pathogenesis. Objectives: This study aimed at detecting the role of interleukin 17 in the pathogenesis of psoriasis. Methodology: This study was a case control study. IL 17 was measured in twenty plaque psoriatic patients in both lesional and non lesional skin and the results were correlated with the disease severity and compared with twenty sex and age matched healthy volunteers that were included as control group. Results: IL 17 was statistically significant higher in lesional than in non lesional areas and also than the control group.