The fatty acid composition of serum phospholipids, cholesteryl esters and triglycerides was measured in 11 obese individuals before and after 2 and 4 weeks treatment with a liquid diet providing about 600 kcal and 70 g protein per day. The patients received 20g of lipid, which was either maize oil and safflower oil (6 patients) or maize oil and evening primrose oil (5 patients). During treatment serum phospholipid linoleic acid (C18:2n-6) concentration remained constant, the dihomo-gamma-linolenic acid (C20:3n-6) concentration decreased by 44 percent in the safflower oil group and by 37 percent in the evening primrose oil group and the arachidonic acid (C20:4n-6) increased by 26 percent in the safflower oil group and by 20 percent in the evening primrose oil group. The increase in phospholipid arachidonate showed a significant positive correlation with total weight loss during the treatment period. These results suggest that during weight reduction there is a increased mobilization of arachidonic acid from tissues and a decreased rate of linoleic acid desaturation and elongation, which was not significantly influenced by providing gamma-linolenic acid in the diet.