Coronary flow reserve was studied in patients with dilated cardiomyopathy. A 3F coronary Doppler catheter was placed in the proximal left anterior descending artery in each of 10 patients with dilated cardiomyopathy (CDM group), seven patients with coronary artery disease that involved only the left anterior descending artery (CAD group), and seven patients with chest pain syndrome and normal hearts (control group). Coronary flow reserve was calculated as the ratio of the maximum mean coronary blood flow velocity after intracoronary administration of papaverine (10 mg) to resting flow velocity (M/R). The time until maximum flow velocity was reached after papaverine administration (Tmax) was also measured. M/R was lower in the DCM (p < 0.001) and CAD (p < 0.001) groups when compared with the control group. Tmax was not abnormal in the DCM group but was prolonged in the CAD group (p < 0.05). In the DCM group, the M/R ratio correlated with the left ventricular end-diastolic pressure (r = -0.69; p < 0.05), the left ventricular end-diastolic volume index (r = -0.7; p < 0.05), the ejection fraction (r = 0.82; p < 0.01), the left ventricular mass (r = -0.7; p < 0.05), and the left ventricular end-diastolic wall stress (r = -0.84; p < 0.001). These results indicate that coronary flow reserve was decreased in patients with dilated cardiomyopathy and that the mechanism of its reduction may differ from that in patients with coronary artery disease.