Epicardial coronary stenosis causes myocardial ischemia; however, the role of coronary microvessels is poorly understood in the pathogenesis of effort angina. We have previously demonstrated that Rho-kinase pathway is substantially involved in coronary arterial hyperconstriction in patients with vasospastic angina and those with microvascular angina. In the present study, we tested our hypothesis that Rho-kinase is involved in coronary microvascular constriction in patients with effort angina. Intracoronary administration of fasudil (300 microg/min for 15 min), a specific Rho-kinase inhibitor, significantly increased oxygen saturation in coronary sinus vein from 37 +/- 3% to 41 +/- 3% (P < 0.05) but not in six age-matched controls (from 42 +/- 3% to 43 +/- 3%, P = NS). Furthermore, the fasudil treatment significantly ameliorated pacing-induced myocardial ischemia in patients with effort angina (magnitudes of symptom: 1.5 +/- 0.6 to 0.6 +/- 0.4, P < 0.01; ischemic ST-segment depression, 1.8 +/- 0.3 to 1.0 +/- 0.2 mm, P < 0.01; percent lactate production, 50 +/- 17% to 0.4 +/- 7%, P < 0.01) without significant hemodynamic changes. These results provide the first evidence that Rho-kinase is substantially involved in coronary microvascular dysfunction associated with myocardial ischemia in patients with effort angina, suggesting that Rho-kinase can be a novel therapeutic target in ischemic heart disease.