['The present study compared microvascular resistance (MR) of viable myocardium in infarct areas with those in reference areas in patients with chronic myocardial infarction (MI).', 'In 27 patients, MR (ratio distal coronary pressure and flow) of reference and viable infarct areas was calculated at baseline and during hyperaemia. H2 15O positron emission tomography (PET) was used to provide myocardial blood flow measurements. In infarct regions, H2 15O PET solely measures flow in viable myocardium, excluding flow in scar tissue. Distal coronary pressure was measured with a pressure wire in the infarct-related and reference artery. The average time between PET study and infarction was 3.3+/-4.4 years. Mean hyperaemic distal coronary pressure was significantly lower in the infarct-related artery. MR varied considerably between patients and was significantly higher in infarct areas at baseline (135+/-38 vs. 118+/-29 mmHg mL min/mL; P<0.05), but not during hyperaemia (39+/-18 vs. 35+/-11 mmHg mL min/mL). The correlation between MR in infarct and reference areas was significant.'] Is hyperaemic microvascular resistance increased in viable myocardium after chronic myocardial infarction?