BACKGROUND: Current consensus statements maintain that endoscopic vein harvesting (EVH) should be standard care in coronary artery bypass graft surgery, but vein quality and clinical outcomes have been questioned. The VICO trial (Vein Integrity and Clinical Outcomes) was designed to assess the impact of different vein harvesting methods on vessel damage and whether this contributes to clinical outcomes after coronary artery bypass grafting. METHODS: In this single-center, randomized clinical trial, patients undergoing coronary artery bypass grafting with an internal mammary artery and with 1 to 4 vein grafts were recruited. All veins were harvested by a single experienced practitioner. We randomly allocated 300 patients into closed tunnel CO2 EVH (n=100), open tunnel CO2 EVH (n=100), and traditional open vein harvesting (n=100) groups. The primary end point was endothelial integrity and muscular damage of the harvested vein. Secondary end points included clinical outcomes (major adverse cardiac events), use of healthcare resources, and impact on health status (quality-adjusted life-years). RESULTS: The open vein harvesting group demonstrated marginally better endothelial integrity in random samples (85% versus 88% versus 93% for closed tunnel EVH, open tunnel EVH, and open vein harvesting; P<0.001). Closed tunnel EVH displayed the lowest longitudinal hypertrophy (1% versus 13.5% versus 3%; P=0.001). However, no differences in endothelial stretching were observed between groups (37% versus 37% versus 31%; P=0.62). Secondary clinical outcomes demonstrated no significant differences in composite major adverse cardiac event scores at each time point up to 48 months. The quality-adjusted life-year gain per patient was 0.11 (P<0.001) for closed tunnel EVH and 0.07 (P=0.003) for open tunnel EVH compared with open vein harvesting. The likelihood of being cost-effective, at a predefined threshold of £20 000 per quality-adjusted life-year gained, was 75% for closed tunnel EVH, 19% for open tunnel EVH, and 6% for open vein harvesting. CONCLUSIONS: Our study demonstrates that harvesting techniques affect the integrity of different vein layers, albeit only slightly. Secondary outcomes suggest that histological findings do not directly contribute to major adverse cardiac event outcomes. Gains in health status were observed, and cost-effectiveness was better with closed tunnel EVH. High-level experience with endoscopic harvesting performed by a dedicated specialist practitioner gives optimal results comparable to those of open vein harvesting.