BACKGROUND:Prevalence rates of invalid baseline scores on computerized neurocognitive assessments for high school, collegiate, and professional athletes have been published in the literature. At present, there is limited research on the prevalence of invalid baseline scores in pre-high school athletes. HYPOTHESIS:Pre-high school athletes assessed with baseline neurocognitive tests would show higher prevalence rates of invalidity than older youth athletes, and those athletes, regardless of age, who were tested in a large group setting would show a higher prevalence rate of invalidity than athletes tested in a small group setting. STUDY DESIGN:Cross-sectional study; Level of evidence, 3. METHODS:A total of 502 athletes between the ages of 10 and 18 years completed preseason baseline neurocognitive tests in “large” or “small” groups. All athletes completed the online version of ImPACT (Immediate Post-Concussion Assessment and Cognitive Testing). Baseline test results that were “flagged” by the computer software as being of suspect validity and labeled with a “++” symbol were identified for analysis. Participants were retrospectively assigned to 2 independent groups: large group or small group. Test administration of the large group occurred off-site in groups of approximately 10 athletes, and test administration of the small group took place at a private-practice neuropsychology center with only 1 to 3 athletes present. RESULTS:Chi-square analyses identified a significantly greater proportion of participants obtaining invalid baseline results on the basis of age; younger athletes produced significantly more invalid baseline scores (7.0%, 17/244) than older athletes (2.7%, 7/258) (chi2 (1) = 4.99; P = .021). Log-linear analysis revealed a significant age (10-12 years, 13-18 years) x size (small, large) interaction effect (chi2 (4) = 66.1; P < .001) on the prevalence of invalidity, whereby younger athletes tested in larger groups were significantly more likely to provide invalid results (11.9%) than younger athletes tested in smaller groups (5.4%), older athletes tested in larger groups (2.7%), and older athletes tested in smaller groups (2.7%). CONCLUSION:Younger athletes tend to exhibit a greater prevalence of invalid baseline results on neurocognitive computerized tests than older youth athletes; the prevalence increases when testing is conducted in a large group and nonclinical setting.