The metabolism of arachidonic acid by mixed suspensions of leukocytes and platelets prepared from peripheral blood has been studied in 20 patients with psoriasis and 21 healthy controls. A lipoxygenase-derived product, identified as 12,20-dihydroxy-5,8,10,14-eicosatetraenoic acid was formed in increased amounts by the cell suspension from the psoriatic patients. This product results from the metabolism of platelet-derived 12-hydroxy-5,8,10,14-eicosatetraenoic acid by the polymorphonuclear leukocyte 20-hydroxylase enzyme. By contrast, synthesis of the cyclo-oxygenase products 12-hydroxy-5,8,10-heptadecatrienoic acid and thromboxane B2 was diminished. Benoxaprofen, which is known to be beneficial in psoriasis, diminished the levels of 12,20-dihydroxy-5,8,10,14-eicosatetraenoic acid formed in vitro.