Vitamin B12 is required in the metabolism of homocysteine. Vitamin B12 deficiency has been implicated in endothelial dysfunction and cardiovascular disease via hyperhomocysteinaemia. However, the association of vitamin B12 and the severity of coronary artery disease has not been studied to date.This study was conducted with the aim of evaluating the relationship between vitamin B12 and SYNTAX score.Medical records of consecutive patients who underwent coronary artery bypass grafting surgery were retrospectively reviewed. The study group consisted of 127 patients. Vitamin B12, other biochemical parameters, clinical and echocardiographic parameters, and SYNTAX score were evaluated for all patients.Patients with vitamin B12 deficiency had a higher prevalence of cardiovascular risk factors such as diabetes mellitus, and history of transient ischaemic attack/stroke and heart failure. The SYNTAX score was significantly higher in patients with vitamin B12 deficiency (29.2 ± 4.9 vs. 22.5 ± 4.5, p < 0.05).In our study, we found a significant relationship between vitamin B12 deficiency and SYNTAX score, demon-strating the severity and complexity of coronary artery disease.