BACKGROUND Recent studies have demonstrated improved diagnostic accuracy for detecting coronary artery disease (CAD) when myocardial blood flow (MBF) is quantified in absolute terms, but there are no uniformly accepted cutoff values for hemodynamically significant CAD. OBJECTIVES The goal of this study was to determine cutoff values for absolute MBF and to evaluate the diagnostic accuracy of quantitative [O-15]H2O positron emission tomography (PET). METHODS A total of 330 patients underwent both quantitative [O-15]H2O PET imaging and invasive coronary angiography in conjunction with fractional flow reserve measurements. A stenosis >90% and/or fractional flow reserve <= 0.80 was considered obstructive; a stenosis <30% and/or fractional flow reserve >0.80 was nonobstructive. RESULTS Hemodynamically significant CAD was diagnosed in 116 (41%) of 281 patients who fulfilled study criteria for CAD. Resting perfusion was 1.00 +/- 0.25 and 0.92 +/- 0.23 ml/min/g in regions supplied by nonstenotic and significantly stenosed vessels, respectively (p < 0.001). During stress, perfusion increased to 3.26 +/- 1.04 ml/min/g and 1.73 +/- 0.67 ml/min/g, respectively (p < 0.001). The optimal cutoff values were 2.3 and 2.5 for hyperemic MBF and myocardial flow reserve, respectively. For MBF, these cutoff values showed a sensitivity, specificity, and accuracy for detecting significant CAD of 89%, 84%, and 86%, respectively, at a per-patient level and 87%, 85%, and 85% at a per-vessel level. The corresponding myocardial flow reserve values were 86%, 72%, and 78% (per patient) and 80%, 82%, and 81% (per vessel). Age and sex significantly affected diagnostic accuracy of quantitative PET. CONCLUSIONS Quantitative MBF measurements with the use of [O-15]H2O PET provided high diagnostic performance, but both sex and age should be taken into account.