Children with high cholesterol levels are especially vulnerable and are at high risk for early onset coronary heart disease (CHD). Endothelial dysfunction, the earliest phase of CHD, is present in children with elevated cholesterol levels as early as 8 years of age. The long term objective of this study is to develop dietary interventions for the prevention and treatment of endothelial dysfunction in children with common lipid disorders including familial hypercholesterolemia (FH) and familial combined hyperlipidemia (FCH). Increasing evidence suggests that the Mediterranean diet and ω-3 fatty acids found in fish have cardioprotective effects. The specific aims of the study are to: determine whether a Mediterranean diet alone or combined with ω-3 fatty acid (eicosapentaenoic acid)improves endothelial function; evaluate the effects of the dietary interventions on lipids and lipoprotein subclasses; evaluate the effects of the dietary interventions on biomarkers for oxidative stress and inflammation. This study is a randomized, double-blind, placebo-controlled clinical trial that includes 34 children (ages 8-17) treated with the Mediterranean diet and ω-3 fatty acid supplements. A dietary educational behavioral intervention will be conducted over 6 months with individual counseling and group sessions. Endothelial function will be measured noninvasively by high resolution ultrasound of the brachial artery at baseline, 6, 12, 18 and 24 weeks. This study is unique because it is a new intervention designed for children at high risk for early CHD with a dietary component and supplementation with ω-3 fatty acids. If effective, this intervention would be broadly applicable in the community and lend valuable insight about dietary therapy to prevent the progression of CHD in hyperlipidemic children.