This study examined 1) whether plasma total Mg (TMg) and ionized Mg (IMg) concentrations in children are reduced by traumatic brain injury (TBI) and 2) whether the extent of reduction correlates with severity of trauma assessed by the Glasgow Coma Scale (GSC) score. This was a prospective cohort study of 98 pediatric patients who had TBI and were admitted through the emergency department. A GCS score was assigned and blood was obtained upon presentation and 24 h later. Plasma was analyzed for TMg and IMg. Patients were grouped into three categories-GCS scores 13-15, 8-12, and <8-to designate mild (n=21), moderate (n=37), and severe (n=40) TBI, respectively. Blood was obtained from 50 healthy children before elective surgery as controls. Control subjects had a TMg and an IMg of 0.94 +/- 0.08 and 0.550 +/- 0.06 mM. TBI patients had an initial TMg and IMg of 0.83 +/- 0.09 and 0.520 +/- 0.05 mM, respectively. Initial TMg for mild, moderate, and severe TBI subgroups (0.87 +/- 0.16, 0.81 +/- 0.15, and 0.83 +/- 0.14 mM, respectively) was reduced from control subjects (p <0.01). IMg was reduced only in the severe TBI subgroup (0.516 +/- 0.07 mM; p=0.016). Twenty-four hours later, TMg remained lower than in control subjects for all subgroups of TBI; however, IMg normalized. TBI in children is associated with a reduction in TMg, whereas IMg decreased only with severe TBI. IMg returned to control values by 24 h despite a continued lower TMg, suggesting mechanisms to maintain IMg. Changes in plasma IMg may serve as a marker for TBI but only over a limited time interval.