The aim of this study was the determination of the pressure-derived collateral fractional flow reserve (FFRcoll) in patients with three vessel disease and chronic occlusion of the right coronary artery undergoing surgical complete revascularization with the off-pump technique. The angiograms of eight patients were preoperatively analysed to quantify collaterality. FFRcoll was determined before any revascularization (FFRcoll 0), and after revascularization of the left coronary arteries, (FFRcoll 1). FFRcoll 0 was compared to the Rentrop grade, to the left ventricular ejection fraction (LVEF), and to FFRcoll 1. No correlation was demonstrated between preoperative Rentrop grade and FFRcoll 0. There was a linear statistically significant correlation between FFRcoll 0 and LVEF (P;ie0.001). No significant variation of the FFRcoll index was observed after performing left coronary artery bypass grafts. Collaterality observed on the coronary angiogram cannot be used as an estimation of the functional collaterality, which can be better appreciated with the LVEF. The absence of variation of FFRcoll before and after left coronary artery revascularization suggests that grafting of the occluded right coronary artery remains justified.