Polyunsaturated fatty acids (PUFA) are a family of lipids including some subgroups identified by the position of the last double bond in their structure. PUFA n-3 include alpha linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), while PUFA n-6 include linoleic acid (LA) and arachidonic acid (AA). Since PUFA n-3 consumption has been shown to be inversely correlated with coronary heart diseases (CHD) incidence, clinical trials have been principally conducted by administering fish oil supplements or purified PUFA n-3. The relationship between dietary PUFA n-3 and CHD is believed to be only partially mediated by their effects on plasma lipoprotein profile. PUFA n-3 have shown to reduce only slightly total and LDL cholesterol, probably as they crowd saturated fatty acids out of diet. Data on HDL cholesterol suggest that PUFA n-3 produce only a small increase in this fraction. The effect of PUFA n-3 supplementation on plasma triglycerides (TG) is much more important, with a reduction of about 25% in normolipidemic subjects and about 50% in hypertriglyceridemic patients. This effect seems to be mediated by an inhibition of hormone-sensitive lipase, and VLDL secretion, and an increase in apo B liver degradation. They also increase lipoprotein lipase activity resulting in a reduction of post-prandial TG. PUFA n-3 might be used as second line therapy, additional or alternative to fibrates and nicotinic acid, in the treatment of severe hypertriglyceridemia. Furthermore, the addition of PUFA n-3 to statin therapy might contribute to normalize TG levels in patients with combined hyperlipidemia.