Traumatic brain injury (TBI) contributes to 50% of all trauma deaths. The mortality rate for children following severe TBI (Glasgow Coma Scale < 9) ranges from 14-24%. There is currently no therapy to reverse the primary injury associated with TBI. Bone marrow precursor cells (BMPC) or bone marrow mononuclear cellular fractions of bone marrow contain mesenchymal stem cells (MSC) and hematopoetic stem cells (HSC). These cells are a component of bone marrow that preferentially migrate to the site of brain injury and differentiate into neurons and cell supporting elements, improving functional outcome in animals. The primary objective of this study is to determine if BMPC harvest and autologous transplantation is safe in children after TBI. The secondary objective is to determine if late functional outcome is improved with BMPC autologous transplantation compared to age and severity matched concomitant controls. Safety will be determined by monitoring cerebral and systemic hemodynamics during harvest and transplantation, neurologic events (seizure, change in GCS, stroke), local site inflammation/injury, and secondary organ injury. Late outcomes will be determined using age-corrected Glasgow Outcome Scores, and a battery of functional outcome measures. In vitro, an aliquot of cells harvested from patients will be studied for labeling with magnetodendrimers as a feasibility study, and these cells will not be reinfused into the patients. The primary endpoint is to assess the safety of autologous BMPC harvest/transplantation in the acute injury phase (hospital stay) and the secondary endpoint is to assess efficacy through 1 and 6 month post-injury follow-up. The rationale for the use of autologous BMPC transplantation is based on a large volume of in vitro and in vivo animal data (see background and significance section). The rationale for using children as the primary population is that children have a greater neurologic plasticity with a unique injury pattern when compared to adults. Children are more likely to have isolated TBI that is more diffuse and less likely to be secondary to extra-axial fluid collections. Patients aged 5-14 years old with GCS of 5-8 will be considered for enrollment into the study. Within 24-36 hours of injury, enrolled patients will undergo bone marrow harvest/BMPC separation and re-infusion. Daily monitoring of the safety outcomes measures and long term neurologic outcomes will be performed. This study should determine if bone marrow harvest, BMPC separation, and reinfusion is safe in children after severe TBI.