['Well-developed collaterals to a myocardial segment supplied by a chronic total occlusion (CTO) and/or left ventricular dysfunction in the CTO regions in patients with chronic stable angina suggest that severe ischemia is unlikely to be present. We evaluated the presence and severity of ischemia using fractional flow reserve (FFR) of the myocardium supplied by a CTO in patients and compared the results with a non-CTO control group.', 'Patients undergoing FFR and successful percutaneous coronary intervention (PCI) of a CTO were evaluated and compared to a matched non-CTO control group.', 'One hundred patients were included (50 CTO/50 controls). CTO lesions were longer (31.6 ± 18.9 vs 20.2 ± 14.9 mm, P = 0.004) and required more stents (2.2 ± 0.8 vs 1.2 ± 0.5, P = 0.001). FFR was lower (P = 0.0003) with CTO (0.45 ± 0.15) than controls (0.58 ± 0.17) prior to intervention but similar after PCI (CTO 0.91 ± 0.05 vs non-CTO 0.90 ± 0.08). All CTO patients demonstrated an ischemic FFR, even with severe regional dysfunction or well-developed collaterals. Resting ischemia was present in 78% (39/50) of CTO patients as evidenced by a resting Pd /Pa of <0.80.'] Is the myocardium supplied by a chronic total occlusion a persistently ischemic zone?