Background: After coronary bypass surgery, occlusion or narrowing of bypass grafts may occur over time. The present study prospectively evaluated the angiographic patency of bypass grafts after 1 year in relation to the preoperative angiographic and functionally severity of the coronary lesion assessed by fractional flow reserve measurement to test the hypothesis that grafting of less critical stenosis may be a risk factor for early dysfunction of the graft. Methods: The study comprised 164 patients eligible for coronary artery bypass surgery who were not suitable for percutaneous intervention and with at least one intermediate lesion. Fractional flow reserve was measured in all lesions to be grafted to establish if a lesion was functionally significant. The surgeon was blinded to the results of these measurements. One year after surgery, coronary angiography was performed to establish bypass graft patency. Results: At coronary angiography after 1 year, 8.9% of the bypass grafts on functionally significant lesions were occluded, and 21.4% of the bypass grafts on functionally nonsignificant lesions were occluded. There was no difference in angina class or repeat interventions between patients with or without occluded bypass grafts. Conclusions: The patency of bypass grafts on functionally significant lesions is significantly higher than the patency of bypass grafts on nonsignificant lesions; however, this finding has no clinical relevance because patients with patent or occluded bypass grafts on nonsignificant lesions did not experience an excess of angina or repeat interventions.