The functional role of collateral circulation in reversible asynergy of the left ventricle was evaluated. Cineventriculograms were obtained before and after the administration of sublingual nitroglycerin (0.3 mg) in 19 patients with complete occlusion of the proximal portion of the left anterior descending coronary artery. In nine patients with well-developed collateral circulation, both left ventricular ejection fraction and regional wall motion were significantly improved by nitroglycerin. By contrast, in 10 patients without significant collateral circulation, there were no detectable changes in both global and regional wall motions before and after nitroglycerin. The left ventricular hemodynamic changes caused by nitroglycerin were comparable in both groups. These findings support the view that the improvement in asynergy caused by nitroglycerin appears to depend mainly on the extent of collateral circulation, rather than on decreases in both preload and afterload.