OBJECTIVE: The effects of concussion on mental status are often difficult to assess on routine clinical examination. I investigated the efficacy of standardized mental status testing on the sport sideline to detect abnormalities that result from concussion and provide an objective measure of postinjury cognitive recovery. DESIGN AND SETTING: All subjects underwent a standardized preseason baseline mental status evaluation. Standardized testing of injured and uninjured control subjects was repeated on the sideline immediately after concussion and 48 hours after injury. SUBJECTS: Sixty-three high school and collegiate football players with concussion and 55 uninjured control subjects were studied. MEASUREMENTS: The Standardized Assessment of Concussion (SAC) was administered to evaluate neurocognitive functioning and neurologic status. RESULTS: Immediately after concussion, injured subjects performed significantly below preinjury baseline and below uninjured controls on the SAC. Measurable deficits in orientation, concentration, and memory were evident immediately after concussion. A decline in SAC score at time of injury was 95% sensitive and 76% specific in accurately classifying injured and uninjured subjects on the sideline. Injured subjects demonstrated significant improvements in SAC score 48 hours after injury. CONCLUSIONS: Standardized mental status testing can be a valuable tool to assist the sports medicine clinician in detecting the immediate effects of concussion on mental status, tracking resolution of immediate postconcussive mental status abnormalities, and making more informed decisions on return to play after injury.