['The release of endothelium-derived relaxing factors has been shown experimentally to be of pivotal importance for the maintenance of coronary blood flow during increased demand. In humans with coronary atherosclerosis, endothelial vasodilator dysfunction is not confined only to epicardial conductance vessels but may also extend into the coronary microcirculation. We therefore tested the hypothesis that endothelial vasodilator dysfunction of the coronary resistance vasculature is associated with myocardial ischemia during exercise in patients without hemodynamically significant epicardial artery stenoses.', 'Coronary vasodilator function was assessed by subselective infusion of the endothelium-dependent dilator acetylcholine (0.036 to 3.6 micrograms/mL) and the endothelium-independent dilator papaverine (7 mg). Coronary blood flow responses were evaluated by intracoronary Doppler flow velocity recordings and quantitative angiography. Exercise-induced myocardial perfusion was determined by 201Tl single photon emission computed tomographic imaging. Thirteen patients had exercise-induced myocardial perfusion defects suggestive of myocardial ischemia, whereas 14 patients had normal thallium imaging during exercise. In patients with exercise-induced thallium perfusion defects, coronary blood flow responses to acetylcholine were significantly (P < .005) blunted compared with patients with normal thallium imaging during exercise. In contrast, coronary blood flow reserve to the endothelium-independent smooth muscle relaxant papaverine was similar in the two groups. Patients with exercise-induced thallium perfusion defects exhibited a significantly (P < .005) reduced (23.9 +/- 9.0% [mean +/- SD]) endothelium-mediated coronary vasodilator capacity compared with patients with normal thallium testing (56.2 +/- 27.8%). Epicardial artery vasoreactivity to acetylcholine did not differ between the two groups.'] Is impaired endothelium-dependent vasodilation of coronary resistance vessels associated with exercise-induced myocardial ischemia?