Context: The Lystedt law requires high school athletes who have sustained a concussion to be removed from practice and play and not to be allowed to return until cleared by a medical professional. Objective: To determine the effect of the Lystedt law on injury and concussion documentation in the Seattle public high schools. Design: Cross-sectional study. Setting: Seattle public high schools. Patients or Other Participants: The numbers of students, aged 13 to 19 years in the 2008-2009, 2009-2010, and 2010-2011 school years, were 4348, 4925, and 4806, respectively. Main Outcome Measure(s): All injuries documented in SportsWare by athletic trainers in Seattle public high schools. We evaluated all injuries, including concussions recorded during the 2008-2009 school year, before the Lystedt law, and during the 2 school years after the law took effect (2009-2010 and 2010-2011). Incidence rates before and after the law were estimated and compared. Results: The concussion rate was -1.09% in 2008-2009, 2.26% in 2009-2010, and 2.26% in 2010-2011. A comparison of relative risks showed that the incidence rates of concussions were different before and 1 year after the Lystedt law (relative risk = 2.10; 95% confidence interval [CI] = 1.50, 2.93) and 2 years after the law (relative risk = 2.10; 95% CI = 1.49, 2.93). Overall, the mean number of days out of play after 2008-2009 was almost 7 days greater after the law took effect (difference = 6.9 days; 95% CI = 0.70, 13.1). For females, the mean number of days out of play after 2008-2009 was more than 17 days in 2009-2010 (difference = 17.2 days; 95% CI = 4.81, 29.5) and was more than 6 days in 2010-2011 (difference = 6.3 days; 95% CI = 1.62, 11.0). Conclusions: The number of documented concussions more than doubled after the institution of the Lystedt law, which may be attributed to heightened awareness and closer monitoring.