BACKGROUND Motor vehicle crashes are the leading cause of death in the United States among persons aged 5-34 years. Seat belts have been shown to be the most effective method for reducing injuries among adults in the event of a crash. METHODS CDC used 2009 data from the National Electronic Injury Surveillance System--All Injury Program (NEISS-AIP) to provide U.S. estimates of the number and rate of nonfatal, motor vehicle--occupant injuries treated in emergency departments among adults aged ≥18 years. In addition, CDC used 2008 data from the Behavioral Risk Factor Surveillance System (BRFSS) to estimate the prevalence of self-reported seat belt use among adults in the United States. Seat belt use was examined further by type of state seat belt enforcement law. RESULTS In 2009 in the United States, an estimated 2.3 million adult motor vehicle--occupants had nonfatal injuries treated in emergency departments. The nonfatal, motor vehicle--occupant injury rate declined 15.6% from 1,193.8 per 100,000 population in 2001 to 1,007.5 per 100,000 population in 2009. In 2008, self-reported seat belt use was higher in states with primary enforcement laws (88.2%), compared with states with secondary enforcement laws (79.2%). If the secondary law states had achieved 88.2% seat belt use in 2008, an additional 7.3 million adults would have been belted. From 2002 to 2008, self-reported seat belt use increased overall from 80.5% to 85.0%. CONCLUSIONS Nonfatal, motor vehicle--occupant injuries treated in emergency departments have declined in recent years but still affect a substantial proportion of the adult U.S. population each year. Self-reported belt use increased from 2002 to 2008, and was higher in states with primary enforcement laws compared with states with secondary enforcement laws. CONCLUSIONS Seat belt use is a proven method to reduce motor vehicle--occupant injuries, and the results of this analysis demonstrate that states with primary enforcement laws have higher prevalence of self-reported seat belt use. To help reduce the number of motor vehicle--occupant injuries, 19 states without primary enforcement laws should consider enacting them.