Coronary sinus blood flow (ml/100 g left ventricular [LV] mass/min) and coronary resistance (mean aortic minus LV mean diastolic pressures/coronary sinus blood flow, mm Hg/[ml/100 g/min]) were studied in 7 control patients and in 11 patients with severe dilated cardiomyopathy (DC) and normal coronary arteriograms. Basal coronary sinus blood flow was not different in the 2 groups. After intravenous administration of dipyridamole (0.14 mg/kg/min X 4 min), coronary sinus blood flow and dipyridamole/basal coronary sinus blood flow ratio were significantly (p less than 0.001) lower in the DC group than in the normal group (coronary sinus blood flow 188 +/- 48 vs 408 +/- 58, respectively; blood flow ratio 1.78 +/- 0.35 vs 4.01 +/- 0.56, respectively), and the coronary resistance was higher in the DC group than in the control group (0.39 +/- 0.15 vs 0.22 +/- 0.03, respectively, p less than 0.01). After administration of dipyridamole in patients with DC, no correlation could be found between coronary sinus blood flow and LV mean diastolic, mean aortic or coronary driving pressures, i.e., mean aortic minus LV mean diastolic pressures. Thus, in DC patients, neither an elevated LV diastolic pressure nor a low coronary perfusion pressure can totally account for the restriction of the coronary flow reserve after dipyridamole.