This monograph discusses a clinical decision-making rule for mild head injury in young children, an extremely common type of injury seen in emergency departments. These children appear to respond differently to mild head injuries and face different developmental issues than do older children. Management guidelines for these types of head injury remain unclear. A classification and regression tree (CART) program was used to identify characteristics that were correlated with intracranial injury (ICI) among children in the study. The information garnered from the tree was used to construct a clinical decision-making rule for the evaluation of very young children with mild head injuries. A cost analysis was done to determine potential cost savings from the new decision-making rule. The authors determined that very young children have unique injury and symptom characteristics that merit further study and may require a separate decision-making process. Minor changes in the way children without ICIs are treated could result in institutional cost savings of as much as $90,000 per year.