Several studies have indicated that certain lipoxygenation products of arachidonic acid, particularly leukotriene B4 (LTB4), may be involved in psoriatic pathophysiology. One way of inhibiting the formation of LTB4 is to replace arachidonic acid in phospholipids with eicosapentaenoic acid. Eicosapentaenoic acid is converted into LTB5, which has a lower biologic activity than LTB4. In the present study psoriatic patients were put on a low-fat diet supplemented with dietary fish oil (Max-EPA 30 ml daily), a source of eicosapentaenoic acid, for 4 months. Twenty-six out of 30 patients with psoriasis vulgaris completed the study. Moderate or excellent improvement was observed in 58% of the patients, while mild improvement or no change was observed in 19% and 23%, respectively. The capacity of peripheral blood neutrophils to synthesize LTB4 and LTB5 in vitro was determined after stimulation with A23187. Before the study, negligible amounts of LTB5 were formed. After 1 month the average of LTB5/LTB4 ratio was 0.42. No further increase of the LTB5/LTB4 ratio was found. There existed no relationship between the clinical response and the LTB5/LTB4 ratio. The results of the present study suggest that dietary fish oil supplementation may be used in the therapy in psoriasis. However, studies defining the dose and the quality of fish oils are imperative.