Fractional flow reserve (FFR)-based coronary interventions of intermediate-severity lesions are safe, cost effective, and have prognostic importance. Although FFR is not affected by heart rate or blood pressure, collateral circulation might affect FFR results. Intermediate stenosis at the donor artery might be overestimated with FFR measurement due to coronary steal. Moreover, the amount of collateral circulation might be a strong determinant of this inaccurate measurement. In this report, we present 8 patients who underwent percutaneous coronary intervention for totally or subtotally occluded recipient vessels that were collateralized by a vessel with intermediate-degree stenosis proximal to the separation of the donor side branch evaluated by quantitative coronary angiography (QCA). In patients with Rentrop grade-2 or grade-3 collateral flow, FFR value of the donor artery was increased at least 0.10 after revascularization of the recipient artery. However, FFR value did not change significantly in patients with Rentrop grade-0 or grade-1 collateral flow following revascularization. In this case series, we suggest that well-developed collateral circulation might result in overestimation of the FFR value in the donor artery with mild stenosis. Therefore, in patients undergoing intervention to the recipient artery with a well-developed collateral supply and an intermediate stenosis at the donor artery, hemodynamic significance of the stenotic lesion should be evaluated not only before but also after coronary intervention. However, if there is no sufficient collateral circulation to totally occluded arteries, FFR values of donor arteries seem to be relatively stable both before and after PCI to the recipient artery.