Introduction: Despite still being the gold standard for anatomical delineation of coronaries, it actually has a limited usefulness to assess the true functional relevance of coronary arterial stenosis. On contrary, Fractional flow reserve “FFR” is an accurate and specific index to determine whether a particular stenosis can be held accountable for ischemia. Aim: We aimed to assess the residual ischemia post percutaneous coronary intervention “PCI” for all ischemic patients either acute or chronic by FFR. Methodology: we recruited 100 patients with significant coronary artery disease (angiographically and FFR<0.8) and planned for PCI at Zagazig University Hospital and National Heart Institute. FFR was performed, only patients with FFR<0.8 were included. The patients were divided randomly into two groups, according to FFR performed or not after stenting, into Group I; 50 patients, after stenting, FFR was done. It was further subdivided into Subgroup A (FFR<0.8) and Subgroup B (FFR>0.8). If FFR<0.8, IVUS had been done to assess the mechanism of residual ischemia then correction of the cause till FFR exceeded 0.8. Group II; 50 patients, after stenting, No FFR was done. All patients were followed up during hospital stay and after 3 months. Results: During hospital Follow up, there was significant difference between both groups regarding chest pain (26% in Group II, versus 4% in Group I) and heart failure (28% in Group II versus 0% in Group I). These results were comparable to three months follow up, chest pain was (41.7% in Group II versus 0% in Group I), heart failure was (20.8% in Group II versus 0% within Group I) and occurrence of sudden cardiac death or arrhythmias (occurred only in Group II; 2.1% and 12.5% respectively). There was also a significant difference concerning in-stent thrombosis, it was recorded in 20.8% within survivors of Group II versus 0% within Group I). In Group I, 14 patients (28% of patients) had FFR< 0.8 after coronary stenting and 36 patients (72% of patients) had FFR>0.8. There was significant difference between both subgroups regarding smoking, diabetes, hypertension (all were higher in those with FFR<0.8 “Subgroup A”), resting ECG findings (Acute STEMI occurred in 27.8% in Subgroup B versus 50% within subgroup A), HbA1c, total Cholesterol and Triglycerides (higher in Subgroup A), HDL-cholesterol (lower in Subgroup A). There was statistically significant positive correlation between stent size and Pre-dilatation FFR. On the other hand, there is a significant negative correlation between stent length and Pre-dilatation FFR. Regarding Multivariate regression, increasing HbA1c, and increasing stent length increase risk of FFR to be<0.8 by 16.402 and 1.356 folds respectively. Increasing stent size protect against FFR< 0.8. Conclusion: Post-PCI FFR has a great potential to direct the quality of PCI, optimize PCI, and improve prognosis by detection and subsequently correction of residual ischemia. Residual ischemia is one of a major factor that cause many post-stenting complications either immediately or during 3 months follow up. Routine FFR guided PCI is associated with less cardiovascular adverse events but is rarely performed in clinical practice due to limited data and lack of any specific guideline recommendations.