During the past 2 decades, the cholesterol coronary artery disease (CAD) hypothesis has become established with considerable attention being directed toward lipid intervention for primary and secondary coronary prevention.1 Although the National Cholesterol Education Program has focused mostly on total cholesterol and low-density lipoprotein (LDL) cholesterol in their assessment and treatment recommendations,2 recent data from epidemiologic, lipid intervention, coronary angiographic studies, and studies of patients with known severe CAD, suggest that a low level of high-density lipoprotein (HDL) cholesterol is perhaps the strongest lipid risk factor for CAD.3-6 In fact, a high percentage of patients with CAD have "isolated" low levels of HDL cholesterol.4 However, nonpharmacologic and drug treatment of patients with isolated low HDL cholesterol has been disappointing. 3,7 Since niacin has beneficial effects on reducing total cholesterol, LDL cholesterol and triglycerides, and also increases HDL cholesterol, it improves the lipid profile in most patients with dyslipidemias. However, the efficacy of niacin therapy in patients with very low levels of HDL cholesterol (e.g., <30 mg/dl), and particularly for isolated low HDL cholesterol, is not established. Therefore, we assessed the efficacy of sustained-release niacin therapy in CAD patients with very low levels of HDL cholesterol, including its effectiveness in a subgroup with isolated low HDL cholesterol compared with a subgroup with hypertriglyceridemia. © 1992.