BACKGROUND: The utility of postconcussion neurocognitive testing versus symptom data has been debated. The sensitivity of the desktop version of the Immediate Post-concussion assessment and cognitive testing (ImPACT) tool has been documented, but psychometric properties of the recently released online version of ImPACT have yet to be fully established. PURPOSE: To document the sensitivity of the online ImPACT version in samples of (1) symptomatic concussed (high school and collegiate) athletes, and (2) asymptomatic concussed (high school and collegiate) athletes suspected of hiding their concussions. STUDY DESIGN: Cohort study; level of evidence, 3. METHODS: A total of 81 athletes observed to sustain a concussion by a certified athletic trainer or team physician, a finding that was confirmed with reported postconcussion symptoms, completed the ImPACT test within 3 days of injury. Data were compared with an independent sample of 81 athletes who completed preseason baseline cognitive assessments using ImPACT and who were matched (with concussed athletes) on the basis of sex, age, sport, concussion history, and absence of attention deficit hyperactivity disorder and learning disability. An independent group of 37 athletes who were also observed to sustain a concussion completed ImPACT within 3 days of injury. These athletes reported no postconcussion symptoms but were noted for suspected invalid response patterns on ImPACT (impulse control index >30 and verbal memory index <69%). The subscale data from the assessments (excluding those contributing to the aforementioned indices) were compared with a matched sample of 37 athletes who completed preseason baseline cognitive assessments in ImPACT (using the same criteria described above). RESULTS: Data from the ImPACT online version yielded 91.4% sensitivity and 69.1% specificity. For asymptomatic athletes suspected of hiding their concussion, data from ImPACT yielded 94.6% sensitivity and 97.3% specificity. CONCLUSION: The online version of the ImPACT tool is a valid measure of neurocognitive performance at the acute stages of concussion, with high levels of sensitivity and specificity, even when athletes appear to be denying postconcussion symptoms.