Background: Computer-based tests are widely used for the purpose of documenting baseline neurocognitive function, and athletes are often tested in groups. At present, there is limited research on the effects of administering these tests in a group versus individual setting. Hypothesis: Athletes assessed with neurocognitive tests in groups would show decreased performance compared with those assessed individually. Study Design: Cohort study; Level of evidence, 3. Methods: High school athletes completed preseason baseline neurocognitive tests either in groups in their school or individually in a private neuropsychological clinic (with no significant between-group differences in sex, history of concussion, and attention deficit disorder/learning disability). All athletes completed the ImPACT (Immediate Post-Concussion Assessment and Cognitive Testing) test battery on 1 occasion, which yielded scores in the area of verbal memory, visual memory, motor processing speed, reaction time, and symptom scores. Results: Athletes in the group setting scored significantly lower on verbal memory (mean 83.4 vs 86.5; P = .003), visual memory (mean 71.6 vs 76.7; P = .0001), motor processing speed (mean 35.6 vs 38.4; P = .0001), and reaction time (mean 0.61 vs 0.57; P = .001), but not symptom scores (mean 6.1 vs 4.4; P = .11), and exhibited a greater rate of invalid baselines. Similar results were obtained after excluding athletes with attention deficit disorder/learning disability and/or a history of concussion. Conclusion: Administering baseline neurocognitive testing to athletes in a group setting may introduce extraneous error, negatively affecting test performance.