The purpose of this study was to compare the relative effects of omega-3 fatty acids (omega-3 FAs) in the triglyceride (TG) and methly ester (ME) forms in a crossover design in patients with type IV hyperlipidemia. Eight male patients were given 18 vegetable-oil capsules (control); 18 capsules of a TG rich in omega-3 FAs (omega-3 TG); and 11 capsules containing omega-3-FA MEs (omega-3 ME). One supplement was given during each of three 6-wk periods. Equivalent amounts of omega-3 FAs (6.8 g/d) were provided by each of the omega-3 treatments. Plasma cholesterol (C) levels were unchanged during the two omega-3 phases whereas plasma TG levels fell by 44% during both. Low-density-lipoprotein cholesterol (LDL-C) levels rose significantly with both omega-3-FA treatments, as did apolipoprotein B levels. When taken in either the TG or ME forms, omega-3 FAs are equally effective hypotriglyceridemic agents but they may raise LDL-C levels.