The relevance of arachidonic acid metabolites in inflammatory skin diseases has been expanded by recent developments in analytical chemistry. The metabolites include prostaglandins, leukotrienes and monohydroxy fatty acids. In ultraviolet light-induced inflammation the concentrations of arachidonic acid, prostaglandin E2, prostaglandin F2 alpha and 6-oxo-prostaglandin F1 alpha are elevated. Indomethacin totally suppresses the evoked prostaglandin increase, but erythema is only partially suppressed. This indicates that prostaglandins are partially involved in erythema production. In psoriasis the first histological change is an infiltration into the epidermis by neutrophilic leucocytes. It has been suggested that chemotactic factors, such as complement derived factors or leukotriene B4 play a role in the pathogenesis of psoriasis.