Background: Fractional flow reserve (FFR), defined as the ratio of maximum flow in the presence of a stenosis to normal maximum flow, is a lesion- specific index of stenosis severity that can be calculated by simultaneous measurement of mean arterial, distal coronary, and central venous pressure (P(a), P(d), and P(v), respectively), during pharmacological vasodilation. The aims of this were to define ranges of FFR values, whether associated with inducible ischemia or not, the to investigate FFR in normal coronary arteries. Methods and Results: In 60 patients accepted for percutaneous transluminal coronary angioplasty (PTCA) of single-vessel disease, with a positive exercise test (ET) <24 hours before PTCA, FFR was determined during adenosine-induced hyperemia just before and 15 minutes after angioplasty. P(a) was measured by the guiding catheter, P(d) by an 0.018-in fiber-optic pressure-monitoring wire, and P(v) by a multipurpose catheter. The ET was repeated after 5 to 7 days, and only if this second ET had reverted to normal was the pre-PTCA value of FFR definitely considered to be associated with inducible ischemia and the post-PTCA value not. Myocardial FFR (FFR(myo)) increased from 0.53±0.15 before PTCA to 0.88±0.07 after PTCA. Coronary FFR increased from 0.38±0.19 to 0.83±0.12. In all patients, values of FFR(myo) definitely associated with ischemia were ≤0.74, whereas all except two values not associated with inducible ischemia exceeded 0.74. Moreover, FFR(myo) in 18 coronary arteries in 5 normal patients equaled 0.98±0.03. Conclusions: A value of FFR(myo) of 0.74 reliably discriminates coronary stenosis, whether associated with inducible ischemia or not. Therefore, FFR(myo) is a useful index to determine the functional significance of an epicardial coronary stenosis and may facilitate clinical decision making in patients with an equivocal coronary stenosis.