In the four years after parliament lowered the alcohol minimum purchase age in 1999, from 20 to 18 years, the rate of traffic crash injuries caused by 15-19 year-old alcohol-impaired drivers was greater than would have been expected if the change had not occurred. Other health outcomes could not be investigated due to lack of statistical power. We will investigate longer-term effects of this important law change on traffic crash injury and assault, two costly health problems. The study will take advantage of the 12 years that have elapsed since the law change (with enhanced statistical power) and will examine changes in injury rates and trends among 18-19 year-olds (targeted by the legislation), 15-17 year-olds (with increased access via older friends and siblings), and three groups (20-24, 25-39, and 40-59 year-olds) not targeted by the law change but also subject to extraneous influences, such as economic conditions. The findings will have direct relevance to public policy.