OBJECTIVE Although duplex vein mapping (DVM) of the great saphenous vein (GSV) is common practice, there is no level I evidence for its application. Our prospective randomized trial studied the effect of preoperative DVM in infrainguinal bypass surgery. METHODS Consecutive patients undergoing primary bypass grafting were prospectively randomized for DVM of the GSV (group A) or no DMV of the GSV (group B) before surgery. Society for Vascular Surgery reporting standards were applied. RESULTS From December 2009 to December 2010, 103 patients were enrolled: 51 (group A) underwent DVM of the GSV, and 52 (group B) did not. Group A and group B not differ statistically in age (72.8 vs 71.1 years), sex (women, 29.4% vs 34.6%), cardiovascular risk factors, body mass index (25.9 vs 26.1 kg/m(2)), bypass anatomy, and runoff. Group A and B had equal operative time (151.4 vs 151.1 minutes), incisional length (39.4 vs 39.9 cm), and secondary bypass patency at 30 days (96.1% vs 96.2%; P = .49). Conduit issues resulted in six intraoperative changes of the operative plan in group B vs none in group A (P = .014). Median postoperative length of stay was comparable in both groups (P = .18). Surgical site infections (SSIs) were classified (in group A vs B) as minor (23.5% vs 23.1%; P = 1.0) and major (1.9% vs 21.2%; P = .004). Readmissions due to SSIs were 3.9% in group A vs 19.2% in group B (P = .028). Two patients in group B died after complications of SSIs. Multivariate analysis identified preoperative DVM as the only significant factor influencing the development of major SSI (P = .0038). CONCLUSIONS Routine DVM should be recommended for infrainguinal bypass surgery. The study found that preoperative DVM significantly avoids unnecessary surgical exploration, development of major SSI, and reduces frequency of readmissions for SSI treatment.