Hypomagnesemia is a common disorder in noncardiac surgical patients in the postoperative period, but the effect of cardiac surgery on serum magnesium concentrations remains unclear. The authors hypothesized that cardiac surgery is associated with hypomagnesemia, and prospectively studied 101 subjects (60 +/- 13.1 years of age) undergoing coronary artery revascularization (n = 70), valve replacement (n = 24), or both simultaneously (n = 7). Blood samples and clinical biochemical data were collected before induction of anesthesia, prior to cardiopulmonary bypass (CPB), immediately after CPB, and on postoperative day 1. Blood samples were analyzed for ultrafilterable magnesium, total magnesium, ionized calcium, parathyroid hormone, and free fatty acid concentrations. Outcome variables were also determined. Eighteen of 99 (18.2%) subjects had hypomagnesemia preinduction and this number increased to 71 of 100 (71.0%) following cessation of CPB (P less than 0.05). Patients with postoperative hypomagnesemia had a higher frequency of atrial dysrhythmias (22 of 71 [31.0%] v 3 of 29 [10.3%], P less than 0.05) and required prolonged mechanical ventilatory support (22 of 63 [34.9%] v 4 of 33 [12.1%], P less than 0.05). Hypomagnesemia is common following cardiac surgical procedures with CPB and is associated with clinically important postoperative morbidity.