OBJECTIVE: Following the Second International Conference on Concussion in Sport in 2005, a summary agreement statement was published that introduced new terminology for sport-related concussions. This new classification system is binary (ie, “simple” versus “complex” concussions). Athletes who are slow to recover (ie, >10 days) are classified as having complex concussions. The purpose of this study was to determine if high school football players, retrospectively classified as having a simple or a complex concussion, could be differentiated in the first 48 after injury on the basis of symptom reporting or neuropsychological testing. DESIGN: Case-control study. SETTING: Pennsylvania high school football programs. PARTICIPANTS: The total sample consisted of 114 concussed high school football players who were identified through a 3 year prospective cohort study. INTERVENTIONS: All completed a computerized neuropsychological screening evaluation within 72 hours of injury. They were followed clinically until they recovered and were cleared to return to play. They were classified retrospectively as having a simple (n = 55) or complex (n = 59) concussion based on their recovery times. MAIN OUTCOME MEASUREMENTS: Neurocognitive test performance and symptom ratings. RESULTS: Within 72 hours after injury, athletes with complex concussions performed more poorly on neuropsychological testing and reported more symptoms than those with simple concussions. Athletes with complex concussions who were slow to recover were 18 times more likely to have 3 unusually low neuropsychological test scores than those with simple concussions (95% CI = 2.3-144.9). Athletes with previous concussions did not recover more slowly. CONCLUSIONS: This study provides evidence that supports and refutes the clinical usefulness of the new simple-complex concussion classification system.