Objective: Very few studies have examined the validity of baseline, preseason neurocognitive test data in student athletes. The purpose of this study was to examine factors that may be associated with invalid baseline testing in a large cohort of high school athletes. Method: Participants were high school athletes (13–18 years of age) who completed baseline ImPACT® testing prior to participating in sports between 2009-2013 (N = 33,732). Chi-square and odd ratios were used to examine the characteristics of high school athletes with valid and invalid baseline tests, as determined by the five ImPACT® validity indicators. Results: Of the total sample, 6.4% (n = 2,163) of high school students had invalid baseline ImPACT® tests. Boys (7.0%) had invalid ImPACT® baseline tests more frequently than girls (5.7%; p < .0001). High school students with self-reported attention deficit hyperactivity disorder (ADHD) had invalid scores more frequently than those without ADHD (10.3% versus 6.1%; p < .0001). Participants with a self-reported learning disorder (LD) had invalid scores more frequently than those without LD (15.3% versus 6.1%; p < .0001). Of the five ImPACT® invalidity indicators, the most commonly flagged indicator was a total letters correct score of less than 8 on the Three Letters test (i.e., 53.9% of high school students with invalid tests met this criterion). Conclusion: The vast majority of high school student athletes had valid baseline tests. Boys and students with ADHD or LD were slightly more likely to have invalid scores.