Hemodynamic function and overall coronary blood flow (argon technique) were measured in 16 patients with idiopathic dilated cardiomyopathy (IDC) and in 12 patients without detectable heart disease (control subjects) referred for precordial pain. In patients with IDC, coronary blood flow was normal at rest (78 ± 17 ml/100 g·min versus 78 ± 9 in control subjects). During maximal inducible coronary vasodilation (dipyridamole, 0.5 mg/kg), coronary blood flow was significantly reduced (142 ± 38 ml/100 g · min versus 301 ± 64 in control subjects; p < 0.001). Consequently, obtainable minimal coronary resistance was increased in IDC (0.54 ± 0.20 mm Hg/ml/100 g · min versus 0.23 ± 0.04 in control subjects; p < 0.001). In patients with IDC, left ventricular (LV) end-diastolic pressure was significantly increased (19 ± 11 mm Hg versus 6 ± 3 in control subjects; p < 0.005), and the LV ejection fraction was diminished (36 ± 11% versus 72 ± 3% in control subjects; p < 0.001). In patients with IDC, LV end-diastolic pressure correlated significantly with the obtained minimal coronary resistance after application of dipyridamole (r = 0.85; p < 0.001). LV catheter biopsy specimens revealed no alterations in myocardial microvasculature. Thus, coronary dilatory capacity is impaired in patients with IDC, due partially to an increase in extravascular component of coronary resistance.