OBJECTIVE We determine the effect of the duration of emergency department (ED) observation on computed tomography (CT) rate for children with minor blunt head trauma. METHODS We performed a prospective cohort study of children with blunt head trauma and a Glasgow Coma Scale score greater than 14. We defined time from injury as the time from head injury to initial physician (emergency attending physician or fellow) assessment. For children who were observed in the ED before CT decisionmaking, we defined ED observation time as time from initial physician assessment to the decision whether to obtain a CT. After adjusting for time from injury, patient age, sex, physician type, and study month, we measured the effect of ED observation time on CT rate in each of the 3 Pediatric Emergency Care Applied Research Network Traumatic Brain Injury risk groups. RESULTS Of the 1,605 eligible patients, we enrolled 1,381 (86%). Of the enrolled patients, 676 (49%) were observed in the ED and 272 (20%) had a CT performed. After adjustment, every hour of ED observation time was associated with a decrease in CT rate for children in all 3 traumatic brain injury risk groups: high risk (adjusted odds ratio [OR] 0.11; 95% confidence interval [CI] 0.05 to 0.24), intermediate risk (adjusted OR 0.28; 95% CI 0.21 to 0.36), and low risk (adjusted OR 0.47; 95% CI 0.31 to 0.73). All 8 children with a significant traumatic brain injury had an immediate CT. CONCLUSIONS For children with minor blunt head trauma, ED observation time was associated with a time-dependent reduction in cranial CT rate, with no delay in the diagnosis of a significant traumatic brain injury.