CONTEXT: Limited data are available regarding the prevalence and nature of invalid computerized baseline neurocognitive test data. OBJECTIVE: To identify the prevalence of invalid baselines on the desktop and online versions of ImPACT and to document the utility of correcting for left-right (L-R) confusion on the desktop version of ImPACT. DESIGN: Cross-sectional study of independent samples of high school (HS) and collegiate athletes who completed the desktop or online versions of ImPACT. Participants or Other Participants: A total of 3769 HS (desktop = 1617, online = 2152) and 2130 collegiate (desktop = 742, online = 1388) athletes completed preseason baseline assessments. Main Outcome Measure(s): Prevalence of = ImPACT validity indicators, with correction for L-R confusion (reversing left and right mouse-click responses) on the desktop version, by test version and group. Chi-square analyses were conducted for sex and attentional or learning disorders. RESULTS: At least 1 invalid indicator was present on 11.9% (desktop) versus 6.3% (online) of the HS baselines and 10.2% (desktop) versus 4.1% (online) of collegiate baselines; correcting for L-R confusion (desktop) decreased this overall prevalence to 8.4% (HS) and 7.5% (collegiate). Online Impulse Control scores alone yielded 0.4% (HS) and 0.9% (collegiate) invalid baselines, compared with 9.0% (HS) and 5.4% (collegiate) on the desktop version; correcting for L-R confusion (desktop) decreased the prevalence of invalid Impulse Control scores to 5.4% (HS) and 2.6% (collegiate). Male athletes and HS athletes with attention deficit or learning disorders who took the online version were more likely to have at least 1 invalid indicator. Utility of additional invalidity indicators is reported. CONCLUSIONS: The online ImPACT version appeared to yield fewer invalid baseline results than did the desktop version. Identification of L-R confusion reduces the prevalence of invalid baselines (desktop only) and the potency of Impulse Control as a validity indicator. We advise test administrators to be vigilant in identifying invalid baseline results as part of routine concussion management and prevention programs.