Familial hypercholesterolemia is associated with premature coronary heart disease. In patients with familial hypercholesterolemia, monotherapy with hydroxymethylglutaril coenzyme. A reductase inhibitors rarely achieves the goal of desirable low-density lipoprotein levels. Epidemiological studies suggest that populations with a high dietary intake of marine n3 fatty acids are protected against coronary heart disease. Hepatic synthesis and secretion of very low density lipoproteins are reduced during fish oil supplementation while other effects on lipid and lipoprotein metabolism are controversial. Fourteen patients affected by familial heterozygous hypercholesterolemia on chronic treatment with simvastatin were enrolled in a double blind, placebo controlled, randomized crossover trial that evaluated the effect of fish oil ethyl ester (Esapent, 5.1 g/day) on lipid and lipoprotein serum concentrations. Total cholesterol, low density lipoprotein cholesterol, high density lipoprotein cholesterol, triglycerides, apoprotein B, apoprotein AI, lipoprotein (a) did not show any significant variation during the four week treatment period with fish oil ethyl ester. The present data suggest that the possible favourable influence of fish oil on the progression of atherosclerosis in these high-risk patients might involve mechanisms which are different from lipid metabolism.