The long-chained fatty acids, which are precursors of prostaglandins and leukotrienes, were examined in plasma lipid esters and adipose tissue obtained from 20 male psoriatic patients and 36 matched controls. The fatty-acid composition of the plasma lipid esters (cholesterol esters, triglycerides and phospholipids) and adipose tissue was assayed using thin-layer and gas chromatography. In comparison with healthy controls, the patients' plasma lipid esters contained significantly lower levels of linoleic acid (18:2 m6) and ~-linolenic acid (18:3 ~3), and higher levels of dihomo-~-linolenic acid (20: 3 ~6). In the adipose tissue of the patients, the amount of ~-linolenic acid was significantly decreased, while that of arachidonic acid (20 (4 ~6) was increased. The observed changes were more pronounced in patients with severe psoriasis than in those with a milder form of the disease. Our results suggest that psoriatic patients differ from healthy controls with regard to the distribution of several of the essential long-chained fatty acids involved in the biosynthesis of prostaglandins and leukotrienes. The relevance of these findings to the development of psoriasis remains to be established.