The optimal time point of staged percutaneous coronary intervention (PCI) among patients with acute coronary syndrome (ACS) remains a matter of debate. Quantitative flow ratio (QFR) is a novel noninvasive method to assess the hemodynamic significance of coronary stenoses. We aimed to investigate whether QFR could refine the timing of staged PCI of non-target vessels (non-TVs) on top of clinical judgment for patients with ACS. For this cohort study, patients with ACS from Bern University Hospital, Switzerland, scheduled to undergo out-of-hospital non-TV staged PCI were eligible. The primary end point was the composite of non-TV myocardial infarction and urgent unplanned non-TV PCI before planned staged PCI. The association between lowest QFR per patient measured in the non-TV (from index angiogram) and the primary end point was assessed using multivariable adjusted Cox proportional hazards regressions with QFR included as linear or penalized spline (nonlinear) term. QFR was measured in 1093 of 1432 patients with ACS scheduled to undergo non-TV staged PCI. Median time to staged PCI was 28 days. The primary end point occurred in 5% of the patients. In multivariable analysis (1018 patients), there was no independent association between non-TV QFR and the primary end point (hazard ratio, 0.87 95% CI, 0.69-1.05 per 0.1 increase; =0.125; nonlinear =0.648). In selected patients with ACS scheduled to undergo staged PCI at a median of 4 weeks after index PCI, QFR did not emerge as an independent predictor of non-TV events before planned staged PCI. Thus, this study does not provide conceptual evidence that QFR is helpful to refine the timing of staged PCI on top of clinical judgment. URL: https://www.clinicaltrials.gov; Unique identifier: NCT02241291.