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Background: Despite the clinical importance of coronary physiological assessment, the relationship between coronary flow reserve (CFR) and fractional flow reserve (FFR) remains poorly defined. Hypothesis: We aimed at studying the relationship between absolute CFR and FFR in patients with stable coronary artery disease (CAD). Methods: Twenty-six patients with moderate (30%-70%) pretest likelihood of CAD underwent 82Rb PET/CT cardiac imaging at rest and after pharmacological stress (dipyridamole 0.142 mg/kg/min for 4 min). CFR was considered abnormal if ≤1.74. Patients then underwent coronary angiography and FFR was measured during intracoronary administration of adenosine (200 μg). FFR was considered abnormal if ≤0.8. A total of 30 stenoses were studied. Results: A significant, albeit modest, correlation exists between CFR and FFR (r=0.44, p=0.02). In particular, of the 16 stenoses with a pathological FFR value ≤0.8, only 6 showed a reduced CFR (black dots), while the other 10 had a normal CFR (red dots); in contrast, of the 14 lesions with an FFR value above 0.8, only one showed a reduced CFR (red dot), while the other 13 had a normal CFR (black dots). Conclusions: In this preliminary series, FFR values >0.8, indicating a non-significant stenosis, show an excellent correlation with absolute CFR, while pathological FFR values (