The fatty acid composition of plasma triglycerides, phospholipids, and cholesterol esters have been studied during gemfibrozil (G)‐induced lipid‐lowering treatment in six patients with hypertriglyceridemia. Gemfibrozil caused significant plasma triglyceride reductions from 776 ± 573 to 226 ± 82 mg/dL (P<.001). Gemfibrozil therapy also caused significant changes in the plasma lipid fatty acid composition, mainly by increasing palmitoleic acid (16:1) in triglycerides (2.1–5.6%) and cholesterol esters (2.2–3.7%), concomitant with the significant decrease in the content of the linoleic acid (18:2) in phospholipids (20.1–16.0%) and triglycerides (18.0–15.2%). The ratio of unsaturated fatty acids (20:3 + 20:4/18:2) was increased in the phospholipid fraction. These findings support a G effect on the desaturation of fatty acids in patients with hypertriglyceridemia. Because fatty acid composition was significantly altered by G, those biologic systems dependent on the pattern of fatty acids may be modified in clinical i