SUMMARYThe metabolism of endogenous arachidonic acid by human polymorphonuclear leukocytes (PMNL) isolated from peripheral blood has been studied in 19 patients with chronic plaque psoriasis and 19 healthy controls. Using calcium ionophore A23187 as a stimulus, the PMNL synthesized leukotriene B4(LTB4), 6‐trans‐leukotriene B4, 12‐epi‐6‐trans‐LTB4, and 5‐hydroxy‐6,8,11,14‐eicosatetraenoic acid. There was no significant difference in the amounts of the products formed between the psoriatic and control groups. The elevated levels of LTB4that have been described in psoriatic skin may therefore be due to the PMNL infiltrate or to enhanced synthesis by another cell type. The reported increase in activity of the circulating PMNL in psoriasis does not appear to be due to increased 5‐lipoxygenase acti