['We sought to examine the relationship between coronary flow reserve (CFR) and myocardial capillary density (MCD) in patients with idiopathic dilated cardiomyopathy, heart failure, and normal coronary arteries.', 'Coronary flow reserve is depressed in patients with idiopathic dilated cardiomyopathy, particularly in those with end-stage congestive heart failure.', 'We studied 18 patients, 48 +/- 10 years of age, who had a mean New York Heart Association functional class of 2.9 +/- 1.3, mean left ventricular ejection fraction of 22 +/- 8%, and mean pulmonary capillary wedge pressure of 23 +/- 10 mm Hg. CFR measurements were made with a 0.014-inch pressure-temperature sensor-tipped guide wire placed in the distal left anterior descending coronary artery. Thermodilution curves were constructed in triplicate at baseline and during maximum hyperemia induced by intravenous adenosine. CFR was calculated from the ratio of mean transit times. Right heart endomyocardial biopsies were performed during the same procedure. Autopsied specimens from nonfailing hearts were used as controls. The tissue was histochemically stained with CD-34 for morphometric measurements of MCD.', 'We observed a close linear relationship between CFR and MCD (r = 0.756, p = 0.0001). The MCD in 7 patients with a CFR >or=2.5 (73.2 +/- 16) was similar to that measured in normal control patients, (85 +/- 11, p = NS). In contrast, the MCD in 11 patients with a CFR <2.5 was 33.2 +/- 14, which was significantly lower than in patients with heart failure and normal CFR (73.2 +/- 16, p = 0.001) or in controls (85 +/- 11, p < 0.0001).'] Is depressed coronary flow reserve associated with decreased myocardial capillary density in patients with heart failure due to idiopathic dilated cardiomyopathy?