Background. The use of one internal thoracic artery has long been considered a gold standard for coronary artery bypass grafting. However, despite the benefits, the frequency of using both internal mammary arteries remains low. The relationship between improving coronary artery bypass grafting results and the risk of complications after surgery using two internal thoracic arteries are yet to be resolved.Aim. We compared immediate and mid-term outcomes of coronary artery bypass grafting using one and two mammary arteries.Methods. Two hundred and thirty-one (231) patients with coronary artery disease, after microscope-assisted coronary artery bypass surgery, were categorised into two groups: group I — patients had received one internal mammary artery during surgery (n = 177), and group II — patients had received two internal mammary arteries during surgery (n = 54). Propensity-score matching was applied to reduce differences between groups, therefore two groups of 50 patients were obtained after matching.Results. Operations with bilateral internal mammary grafting in comparison to single internal mammary grafting were associated with longer operating times (group I: 167.5 [150-190] min., group II: 205 [190–220] min., p < 0.001) and increased aortic cross-clamp times (group I: 37.5 [32–45] min., group II: 44 [39–48] min., p = 0.012), but cardiopulmonary bypass duration was similar (group I: 65.5 [56–78] min. and group II: 69 [58–78] min., p = 0.95). No differences were observed for adverse event frequencies (i.e. death, myocardial infarction, acute cerebrovascular accidents and repeated revascularisation) and sternal wound complications in the early- and long-term post-operation. The grafts patency in groups I and II before discharge was 98.0 vs. 92.5 % (p = 0.195) and 94.6 vs. 92.5 % (p = 0.582) for mammary and venous grafts, respectively. At follow-up time (29.5 ± 18.7 months), group I was associated with significantly improved actuarial overall survival (p = 0.021) and were composite survival free from cardiac-related mortality, myocardial infarction, repeat revascularisation and stroke (p = 0.008). Complete revascularisation was associated with a reduced risk of major cardiac events (OR, 0.41; 95% CI: 0.20–0.85).Conclusion. Bilateral internal mammary artery coronary artery bypass grafting in comparison with single internal mammary grafting was associated with increased operation times, similar grafts patency, early and late outcomes, but no overall improved survival and composite freedom from major adverse cardiac events in the mid-term. Complete revascularisation may be considered an independent prognostic factor after coronary bypass surgery and may be more clinically relevant than both internal thoracic arteries. Received 29 April 2020. Revised 8 September 2020. Accepted 9 September 2020. Funding: The study did not have sponsorship. Conflict of interest: Authors declare no conflict of interest.