Postoperative arrhythmias are among the most common complications of cardiac surgery. Total serum magnesium concentration will change after coronary bypass surgery but compensatory prophylactic administration of magnesium has remained a controversial issue. We studied whether prophylactic administration of magnesium could prevent post-coronary artery bypass grafting (CABG) arrhythmias and evaluated the effects of diabetes mellitus on prophylactic magnesium administration. In a clinical trial, 345 consecutive CABG candidates were randomly assigned to study (n = 166, 48.1%) and control groups. Patients in study group received supplemental magnesium infusion as following: 2 g [corrected] after induction of anesthesia until cardio-pulmonary bypass and then 8 g upon arrival in Intensive Care Unit (ICU) until 24 hr. Total serum magnesium concentration was measured at four designated time points: onset of induction, and 0, 24 and 48 hr after ICU admission. Cardiac arrhythmias were sought with a 12-lead electrocardiogram (ECG) from the end of surgery up till discharge. Atrial Fibrillation (Af) occurred in 34 patients (9.9%). Total serum magnesium concentration was significantly higher in patients who received supplemental magnesium (P < 0.001) and significantly lower in Af patients (P= 0.02). Among non-diabetics, Af incidence was significantly lower in study group compared with control group. The occurrence of atrial fibrillation correlates with serum magnesium level. Diabetes mellitus probably hampers prophylactic effect of supplemental magnesium in preventing the occurrence of Af.