To test the hypothesis that a dietary omega-3 fatty acid, docosahexaenoic acid, improves the lipoprotein subclass profile of children who have hyperlipidemia, we conducted a randomized, double-blind, placebo-controlled study. Children who had hyperlipidemia (n = 20) were stabilized on a low-fat diet for 6 weeks and then randomized to receive 1.2 g/day of docosahexaenoic acid for 6 weeks or placebo. Supplementation with docosahexaenoic acid significantly increased low-density lipoprotein subclass 1 and high-density lipoprotein subclass 2 (large and buoyant; less atherogenic particles) by 91% and 14%, respectively, compared with the placebo phase. Low-density lipoprotein subclass 3 (small and dense; more atherogenic particles) decreased by 48%.