Although in situ internal mammary artery (is-IMA) grafting remains the most frequent conduit in coronary artery bypass grafting (CABG), circumstances may necessitate free grafting of the IMA (f-IMA), though differences in outcomes have not been fully characterized. The purpose of this study was to compare clinical and angiographic outcomes of is-IMA versus f-IMA coronary bypass grafts in patients who underwent elective CABG surgery. In 1,829 patients in the angiographic cohort of PREVENT IV, 1,572 (85.9%) had at least 1 IMA graft; of these, 34 (2.2%) patients had at least 1 f-IMA graft and 1,538 (97.8%) had at least 1 is-IMA graft without additional f-IMA grafts. Characteristics of patients, procedure, and grafts/targets were compared between cohorts. Primary endpoints included death, myocardial infarction, and revascularization, as well as incidence of graft failure (stenosis >75%) on angiography at 12-18 months postoperatively. Patients receiving is-IMA grafts were more often of white race and higher weight. Aortic cross-clamp time was shorter in the f-IMA cohort (39.5 vs 57.0 min, p = 0.04), but duration of bypass was similar (93.5 vs 100.0 minutes, p = 0.793). Of the in situ grafts, 97.3% were via the left internal mammary artery (LIMA), 86.6% were of good quality, and the left anterior descending (LAD) was bypassed in 88.2%. This compares with free grafts, which were via the LIMA in 68.0%, of good quality in 96.1%, and bypassed the LAD in 58.8% and first obtuse marginal (OM1) in 23.5%. Rates of death, myocardial infarction, and revascularization were similar between groups. The rate of graft failure was higher in f-IMA grafts (23.3%) compared with is-IMA grafts (8.5%; p < 0.01). Although clinical outcomes were similar with use of free versus in situ IMA grafts, higher rates of graft failure were encountered with use of the f-IMA graft. In conclusion, in situ grafts should be the preferred conduit for patients who undergo CABG surgery.