The low incidence of coronary heart disease in Greenland Eskimos and Japanese fishermen who have high consumption of fish and seafood has called the attention in the role of eicosapentaenoic (EPA = 20:5n-3) and docosahexaenoic (DHA = 22: 6n-3) acids, the major n-3 polyunsaturated fatty acids (PUFAs) in fish oils, in reducing cardiovascular risk. N-3 PUFAs may reduce serum lipids by promoting fatty acid oxidation and decrease VLDL producing from the liver, depress tissue arachidonate synthesis and alter the composition and tissue PUFAs. Plasma lipoprotein and fatty acid composition of total serum and erythrocyte lipids in 9 hypertriglyceridemic patients consisting of 4 males and 5 females aged 39-72 yr who attended the Nutrition Clinic, Ramathibodi Hospital were investigated. The study period consisted of 4 wks of dietary advice only followed by 48 wks of dietary advice with a daily intake of 6 g of fish oil capsules (FOC). Six grams of FOC provided 1080 mg of 2:5n-3 and 720 mg of 22:6n-3. Their plasma triglyceride (TG) levels at wk 0, 12, 24, 36, 48 were 356.7, 230.1, 209.7, 192.9, 22 7.4 mg/dL and M-particle (equivalent to very low density lipoprotein, VLDL) were 484.4, 354.8, 383.1, 349.7, 453.2 mg/dL respectively that decreased significantly, whereas their plasma low density lipoprotein cholesterol (LDL-C) levels at the same periods were 139.4, 164.9, 171.0, 157.1, 158.3 mg/dL that increased significantly. Serum and erythrocyte 20:5n-3 and 22:6n-3 in these subjects were significantly higher than those at wk 0 throughout the study. These findings indicate the bioavailability of 20: 5n-3 and 22:6n-3 in TG lowering effect of FOC. Fatty acids from fish oil have a remarkable effect on the synthesis and clearance of TG-rich lipoproteins, especially VLDL and chylomicrons. Though daily treatment with 6 g of FOC has a striking effect in increasing plasma LDL-C levels.