Abstract--Excess coronary heart disease morbidity and mortality among African Americans remains an important yet unexplained public health problem. We hypothesized that adverse outcome is in part due to intrinsic or acquired abnormalities in coronary endothelial function and vasoreactivity. We compared dose-response curves relating changes in coronary blood flow and epicardial diameter to graded infusions of acetylcholine in 50 African American and 65 white subjects with hypertensive left ventricular hypertrophy (LVH) and normal coronary arteries. These groups were similar for age, body mass index, mean arterial pressure, and indexed left ventricular mass. The same protocol was conducted in 24 normotensive African American and 56 similar white subjects. We found significant depression in the coronary blood flow dose-response curve relation among African Americans when compared with white subjects with similar LVH (P<0.03). Racial differences were observed at all doses of acetylcholine but were less precisely estimated at the highest dose. The same testing among normotensive subjects revealed similar dose-response curves with no significant effect of race. Qualitatively similar results were found with respect to coronary diameter. Adenosine responses, a measure of endothelium-independent function, were similar after partitioning by LVH. Our study demonstrates that there are racial differences in sensitivity of coronary arteries to acetylcholine-stimulated relaxation among those with LVH. These results provide a mechanism whereby racial differences in coronary vasoreactivity might contribute to adverse coronary heart disease outcome among African Americans, a group in whom LVH is prevalent.