Objective: The purpose of this study is to examine group size effects when collecting baseline or post-injury sports-related concussion data on two computerized neuropsychological screening batteries. Research has suggested that group testing may create group inter-distraction, motivational impairment, or carelessness. Findings will help clinicians better understand group size effects in order to collect the most valid and reliable data. Methods: Immediate Postconcussion Assessment and Cognitive Testing (ImPACT) and Concussion Vital Signs (CVS) were used to measure aspects of cognitive functioning. Participants were 1,132 high school and college student athletes who completed CVS and 642 high school and college students who completed ImPACT during baseline or post-injury assessment in small group sizes (1–3 student athletes) or large group sizes (7–20 student athletes). Average age was 17.8 years (SD ¼ 2.3) and average education was 11.5 years (SD ¼ 1.9). Results: There was a statistically significant effect of group size on the overall test battery mean for ImPACT—F(1, 640) ¼ 4.81, p ¼ .03—and CVS—F(3, 397) ¼ 10.30, p ¼ .001. Overall test battery means for ImPACT were: small group size ¼ 99.6 and large group size ¼ 93.4. Overall test battery means for CVS were: small group size ¼ 93.8 and large group size ¼ 89.1. Conclusions: ANOVA results revealed that the overall test battery means for ImPACT and CVS varied significantly by group size. Discussion of these results and implications for baseline and post-injury sports-related concussion assessment will be presented.