The evidence that plasma levels of apolipoprotein B and apolipoprotein AI are important determinants of the risk of premature coronary artery disease (CAD) are reviewed and a metabolic framework for these relations presented. ApoB and apoAI are compared to total cholesterol and to lipoprotein lipids as predictors of CAD. The emphasis is on apoB as this is the particular interest of this laboratory and because a structural basis has been elucidated that may relate the epidemiologic observations about apoB to the metabolic characteristics of low density lipoprotein. The measurement of apoproteins has revealed much that was previously obscure about normal and abnormal lipoprotein metabolism. However, one of the major challenges is the development of an accurate, widely available commercial assay system for apoproteins.