We experienced 3 cases of serious perioperative coronary artery spasm in off-pump coronary artery bypass grafting (OPCAB). In consideration of the causes, we directed our attention to hypomagnesemia, one of the triggers of coronary artery spasm. This study was performed to confirm the tendency to hypomagnesemia in OPCAB. First, we report 3 patients having severe coronary artery spasm immediately after OPCAB with consideration of the causes. Second, serial magnesium (Mg) value (xylidyl blue method, normal 1.9-3.1 mg/dl) was measured in 45 consecutive patients with OPCAB between April and October 2002, 1) before starting the operation, and 2) after the patient's entrance into the intensive care unit. Preoperative and postoperative values of Mg (mg/dl) were 2.1+/-0.3, 1.7+/-0.3, respectively (p < 0.01). Postoperative incidence of hypomagnesemia was as high as 89% of the patients (40 out of 45 patients). In this study and thereafter, we corrected hypomagnesemia with magnesium sulfate during and after OPCAB, and no perioperative coronary artery spasm occurred. Hypomagnesemia, one of the triggers of coronary artery spasm, is very common in OPCAB. We strongly recommend the correction of hypomagnesemia during and after OPCAB for the prevention of perioperative coronary artery spasm.