In patients with a major coronary vessel occluded, the presence of good coronary collateral circulation increases vulnerability to myocardial ischemia induced by pharmacologic coronary vasodilation. The presence of good collaterals results in a greater frequency, severity, and extent of dipyridamole-induced regional left ventricular dysfunction, consistent with the hypothesis of angiographically assessed collaterals facilitating "steal phenomena" during dipyridamole-induced coronary vasodilation.