OBJECTIVE: To analyze magnesium metabolism in children after cardiac surgery. DESIGN: Prospective observational study. SETTING: Pediatric intensive care unit (PICU) of a university hospital. PATIENTS: A total of 42 children in the perioperative period of cardiac surgery. MEASUREMENTS AND MAIN RESULTS: Determinations of serum magnesium (MgS), ultrafiltrable magnesium (MgU), intraerythrocytic magnesium, urinary magnesium, blood and urinary calcium, phosphorus, sodium, and blood and urinary creatinine were made in the operating room before surgery, at admission to the PICU, at 24 hrs, and at 3 and 5 days after surgery. The relationship with age, sex, diagnosis, type of surgery, arrhythmias, complications, length of stay in the PICU, and mortality was analyzed. MgS and MgU levels decreased after cardiac surgery (MgS before surgery, 0.74 +/- 0.09 mmol/L; MgS at admission to the PICU, 0.66 +/- 0.11 mmol/L; MgU before surgery, 0.56 +/- 0.06 mmol/L; MgU at admission to the PICU, 0.50 +/- 0.07 mmol/L; p <.0001 for both values). At admission to the PICU, 61.5% of the patients had MgS of <0.63 mmol/L, and 47.2% of the patients had an MgU of <0.46 mmol/L. MgS and MgU had increased by 24 hrs and showed further increases over the first 5 days after surgery. There were no significant changes in the intraerythrocytic magnesium levels in the postoperative period. Changes of MgS and MgU were greater after extracorporeal circulation than after closed surgery (p <.001). There was no correlation between MgS, MgU, or intraerythrocytic magnesium and other analytic and clinical parameters. No patient presented arrhythmias and none died. CONCLUSIONS: After cardiac surgery, and particularly after extracorporeal circulation surgery, children present with low MgS and MgU levels at admission to the PICU. MgS and MgU levels increase over the first 5 days after cardiac surgery. No relationship was found between magnesium levels and the postoperative course.