Adolescent idiopathic scoliosis (AIS) patients undergoing posterior spinal fusion (PSF) usually require surveillance in the pediatric ICU (PICU). Some reports have documented evidence of hypomagnesemia following PSF at PICU. Little has been studied about relationship between AIS and postoperative serum magnesium (Mg+2) levels. The aim of this study is to determine the prevalence and risk factors of hypomagnesemia in AIS patients admitted to PICU after PSF. IRB approved the cross-sectional study of AIS patients admitted to PICU after PSF from January 2016 to May 2017. Serum electrolyte levels of phosphorous, calcium, sodium, potassium and hemoglobin were taken preoperative and 24 h postoperative and compared with postoperative magnesium levels. Blood volume reinfused from cell saver return (CSR), estimated blood loss (EBL), volume of fluids intraoperative and average of vertebra levels operated were charted and compared with postoperative magnesium levels. A total of 46 AIS patients undergoing PSF met inclusion criteria, with an average age of 15 years (78% female). Postoperatively, hypomagnesemia was reported in 24/46 patients (53%).No association was found between hypomagnesemia and serum electrolyte (phosphorous, calcium, sodium and potassium) or hemoglobin levels. In addition, no statistical association was found with age, operative time, vertebrae level fused, CSR, EBL and volume of fluids intraoperatively. Fifty-three percent of pediatric patients developed hypomagnesemia after surgical correction for AIS. Further investigation of this electrolyte disturbance will likely result in a useful clinical tool for physician in the management of AIS.