Background Low levels of high-density lipoprotein (HDL) cholesterol increase the risk of coronary artery disease (CAD), and recent clinical studies suggest that interventions in low-HDL patients are beneficial. The purpose of this study was to examine the effect of increased HDL levels on endothelium-dependent vasodilation. Methods We studied patients with CAD with a low-density lipoprotein (LDL) level of <100 mg/dL. Patients with an HDL level of ≤36 mg/dL were treated with niacin (n = 11), and patients with an HDL level of >36 mg/dL were followed as controls (n = 10). Baseline and 3-month follow-up studies of flow-mediated dilation (FMD) and blood lipid levels were obtained. Results HDL levels increased from 30.1 ± 1.2 to 40.5 ± 1.2 mg/dL in the niacin-treated patients (P < .001) but remained unchanged in the control patients. At baseline, FMD was impaired in both the treated (6.5% ± 1%) and the control (7.3% ± 1%) patients compared with 10 healthy subjects (16% ± 2%, P < .01). After 3 months, FMD improved in the niacin-treated patients (11.8% ± 1%, P = .001) but remained unchanged in the control patients (6.2% ± 1%). Exposure of cultured human vascular endothelial cells to HDL in vitro enhanced expression of endothelial nitric oxide synthase (eNOS), as shown by immunoblotting. Conclusions In patients with CAD and well-controlled LDL levels, elevation of HDL with niacin improves endothelial function. HDL increases eNOS protein expression in cultured vascular endothelial cells. Taken together, these observations suggest that HDL-mediated increases in eNOS expression may contribute to the observed enhancement in vasorelaxation and thus support a previously unrecognized mechanism for the beneficial cardiovascular effects of HDL. (Am Heart J 2002;144:165-72.)