A change in reaction time is one of various clinical measures of neurocognitive function that can be monitored after concussion and has been reported to be among the most sensitive indicators of cognitive impairment. To determine the timeline for clinically assessed simple reaction time to return to baseline after a concussion in high school athletes. Observational study. Athletic training room. Twenty-one high school-aged volunteers. Participants completed 8 trials of the ruler-drop test during each session. Along with baseline measures, a total of 6 additional test sessions were completed over the course of 4 weeks after a concussion (days 3, 7, 10, 14, 21, and 28). The mean reaction times calculated for all participants from each of the 7 test sessions were analyzed to assess the change in reaction time over the 7 time intervals. After a concussion and compared with baseline, simple reaction time was, on average, 26 milliseconds slower at 48 to 72 hours postinjury (P < .001), almost 18 milliseconds slower on day 7 (P < .001), and about 9 milliseconds slower on day 10 (P < .001). Simple reaction time did not return to baseline levels until day 14 postinjury. Clinically assessed simple reaction time appeared to return to baseline levels within a timeframe that mirrors other measures of cognitive performance (approximately 14 days).