Researchers disagree on whether plasma triglyceride levels are an independent risk factor for atherosclerotic coronary artery disease. We hypothesized that patients with endogenous hypertriglyceridemia would differ: Some would have normal values of plasma low-density lipoprotein (LDL) B protein; others, despite their normal level of LDL cholesterol, would have increased levels of LDL B protein. We believed the latter patients—those with hyperapobetalipoproteinemia—would be the ones at risk for atherosclerosis. We studied two populations. Group 1, consisting of 162 patients with type IV lipoprotein patterns, was divided into two groups. One subgroup (A), which included 38 patients with elevated plasma LDL B protein levels, had a significantly higher prevalence of atherosclerotic disease than the other subgroup (B) of 36 patients with normal levels of plasma LDL B protein (10 patients versus two, p < 0.02). Group 2 consisted of 100 patients who had had myocardial infarction. Eighty-one percent of the 47 hypertriglyceridemic and 70% of the 53 normotriglyceridemic patients had elevated plasma LDL B protein levels (129 mg/dL or greater)—a proportion significantly higher than that in Group 1 (p < 0.001). Thus, an elevated plasma level of LDL B protein not only identifies subgroups of patients with type IV lipoprotein patterns, but also may be an important marker for atherosclerotic disease.