Background Post-percutaneous coronary intervention (PCI) fractional flow reserve (FFR) is an established method to evaluate the functional result of angiographically successful PCI; however, it is rarely measured. Quantitative flow ratio (QFR) is shown to be correlated with FFR before PCI. In this observational study we sought to evaluate the correlation and relative prognostic value of post-PCI QFR compared with post-PCI FFR.Methods and Results We analyzed the outcome of all patients who underwent FFR-guided PCI with post-PCI FFR measurement. Post-PCI QFR was calculated offline, blinded to the value of post-PCI FFR and the clinical outcome. The primary end point was target-vessel failure (TVF) defined as the composite of cardiovascular death, target-vessel-related nonfatal myocardial infarction, and target-vessel repeat revascularization. The secondary end point was the composite of cardiovascular death and myocardial infarction. Median follow-up was 50 months. QFR calculation was unsuccessful in 15.6% of the vessels; thus, 422 vessels of 365 patients were included in the analysis. Post-PCI QFR systematically overestimated post-PCI FFR with a mean bias of +0.052, and their correlation was poor (Pearson’s r=0.424). Post-PCI FFR was an independent predictor of TVF and cardiovascular death/myocardial infarction. Post-PCI QFR was associated with TVF, but was not an independent predictor of either TVF or cardiovascular death/myocardial infarction. The optimal cutoff of post-PCI FFR to predict TVF was 0.83, whereas no optimal cutoff of post-PCI QFR could be found.Conclusions Post-PCI QFR has a poor correlation with and overestimates post-PCI FFR. Whereas post-PCI FFR is an independent predictor of clinical outcome, post-PCI QFR is not.