To determine the frequency of hypomagnesemia in pediatric patients after spinal fusion, to verify whether postoperative magnesium levels were lower than preoperative levels and, if so, to identify possible causes and assess the clinical repercussions for patients.This was a retrospective descriptive study of pediatric patients admitted to a pediatric intensive care unit (ICU) after spine fusion surgery, between March 1 and August 31, 2011. Preoperative magnesium, phosphorus and total and ionized calcium concentrations were compared with the results of tests conducted during the first 24 hours after admission to the ICU.A total of 45 patients were enrolled on the study. Median age was 13.1 years. Preoperative mean serum magnesium was 1.8 ± 0.2 mg/dL and postoperative serum magnesium was 1.4 ± 0.2 mg/dL, which was a significant reduction between the two periods (p < 0.001). The frequency of hypomagnesemia rose from 1 patient (2%) in the preoperative period to 31 patients (68%) during the postoperative period. There were also significant reductions in concentrations of phosphorus (p < 0.001) and total calcium (p < 0.001). There was a significant correlation between magnesium reductions and the volume of fluids administered during the surgery (p = 0.03), transfused blood volume (p < 0.001) and number of vertebrae fused (p < 0.05). Seven of the 31 patients with hypomagnesemia exhibited symptoms (22%).There was an elevated frequency of hypomagnesemia in patients who underwent spinal fusion. Serum magnesium should be assayed when patients are admitted to the pediatric ICU, so that appropriate supplementation can be initiated immediately, minimizing the risk of complications.