Sports rule/policy changes designed to reduce the incidence of injuries have the potential to have a sizeable effect, reach a large population, with little to no cost. PURPOSE: To investigate the effectiveness of rule/policy changes on injury rates in 3 collegiate sports. METHODS: The National Collegiate Athletic Association Injury Surveillance System (ISS) conducts ongoing, annual surveillance of injuries using a sample of all member schools. Data from 16 academic years were analyzed to assess the effect of rule/policy changes on incident injury rates. Injuries were defined as: 1) occurring from sport participation, 2) required medical treatment, and 3) restricted the athlete's participation for 1 or more days. An exposure was defined as 1 athlete participating in 1 practice/game where there was a risk of injury. 3 specific sport rule/policy changes were evaluated: 1) Field hockey - 1996-1997 offsides/corner shot rule, 2) Baseball - 1999-2000 bat design rule, and 3) Football - 1997-1998 Spring practice contact/equipment policy. Injury rates (IR) per 1000 athlete-exposure (AE) and 95% confidence intervals (CI) were calculated and compared for two time periods: pre and post rule/policy change. Significance was defined as non-overlapping CIs or p< 0.05. RESULTS: For Field Hockey, the overall game injury rate significantly declined from pre (IR 8.65, CI 8.03-9.28) to post (IR 6.91, CI 6.29-7.53); a 35.1% reduction in the absolute number of injuries. For baseball, the IRs from batted balls did not significantly change for all players (pre-IR = 0.53, CI 0.45-0.61; post-IR = 0.60, CI 0.51-0.69) or for pitchers only (pre-IR = 0.15, CI 0.11-0.20; post-IR =0.18, CI 0.13-0.23); a slight increase (5.5% all players, 8.3% pitchers) in the absolute number of batted ball injuries. For Spring football practice, there was a small, but significant (p<0.01), reduction in overall injury rates from pre (IR 9.87, CI 9.61-10.13) to post (IR 9.37, CI 9.12-9.62); a 6.0% reduction in the absolute number of injuries. CONCLUSIONS: Simple changes in sport rules and/or policies may be an effective and wide-reaching intervention to prevent sports injuries, however, changes may not always result in the desired effect. Therefore it is critical to evaluate rule and policy changes in sports to assess both wanted and potentially unwanted consequences.