Plasma levels of lipids, lipoproteins, and apoproteins in 281 patients undergoing cardiac catheterization were correlated with the incidence and severity of coronary artery disease (CAD) to determine if measurements of apoprotein levels are more predictive of the presence and severity of CAD than the corresponding levels of lipoprotein lipids. In 156 men with CAD among 194 men in the study the only variable other than age that correlated with the severity of CAD, defined by the number of lesions and percent stenosis, was the ratio of apoprotein AI to apoprotein B (r = .1908, p < .03). The ratio of apoprotein AI to apoprotein B was a more accurate predictor of the severity of CAD than was the ratio of the corresponding high-density to low-density lipoprotein levels (coefficients of partial determination of .07 and .035; p < .001 and p < .07, respectively). Multivariate analysis confirmed the independent effect of the ratio of apoprotein AI to apoprotein B on the severity of CAD even after adjustments were made for lipid levels, age, presence of hypertension or diabetes, and therapy with β-blockers or diuretics. Among men with total occlusion of a coronary artery apoprotein E and apoprotein B levels were significantly higher than in control subjects with a similar extent of CAD (p < .03). The lipid profiles of the 37 women with CAD were very different from those of the men. In women, only total triglycerides and apoprotein B levels correlated with the severity of disease (r = .3670, p < .03 and r = .3635, p < .04, respectively), as confirmed by multivariate analysis. The results of this study indicate that levels of certain apoproteins may be more accurate predictors of the severity of CAD than are the corresponding levels of lipoprotein lipids, in keeping with the pivotal roles of apoproteins in normal and abnormal lipid metabolism. However, lipids and apoproteins are both weak predictors of lesion severity and cannot be used in the diagnosis of CAD.