OBJECTIVES: This study aimed to develop and validate prognostic criteria to identify children at risk for persistence of mild traumatic brain injury (MTBI) impairment. METHODS: A prospective cohort study was conducted among 11- to 17-year-old emergency department (ED) patients admitted for MTBI. The Immediate Postconcussion Assessment and Cognitive Testing neurocognitive test was administered during hospitalization and at routine clinic follow-up (ImPACT). Logistic regression and receiver operating characteristic (ROC) analyses were used to develop prognostic criteria for MTBI-related impairment in 1 group and validate the criteria in a second group. Mild traumatic brain injury-related impairment was defined as any impairment (symptom score >8 or <25th percentile on at least 1 of 4 neurocognitive composite domains) or severe impairment (symptom score >12 or <25th percentile on at least 2 of 4 neurocognitive composite domains) present on follow-up. RESULTS: The derivation and validation cohorts were 42 and 21 patients (median age, 14 years; 71.4% male). Using the mean of the validation cohort patients' 4 neurocognitive deficit composite percentiles at baseline, a cut point of less than 39 percentile had high sensitivity (0.89) and specificity (0.80) and an area under the ROC curve of 0.85 in predicting the presence of any impairment at follow-up; it discriminated equally well in the validation cohort. A cut point of less than 27 percentile had good sensitivity (0.67) and specificity (0.67) and area under the ROC curve of 0.67 in predicting the presence of severe impairment in the derivation cohort at follow-up; it discriminated equally well in the validation cohort. CONCLUSIONS: This is the first study demonstrating prognostic criteria that may greatly help physicians identify patients who would benefit from structured follow-up care after MTBI.