This surveillance report, prepared by the Alcohol Epidemiologic Data System (AEDS), National Institute on Alcohol Abuse and Alcoholism (NIAAA), presents trends in underage drinking by youth aged 12-20 years between 1991 and 2003. This is the first of a series of reports to be published biennially on underage drinking and related attitudes and risk behaviors. Data for this series are compiled from three separate nationally representative surveys, the National Survey on Drug Use and Health (NSDUH), the Monitoring the Future (MTF) survey, and the Youth Risk Behavior Survey (YRBS). The report highlights use prevalence, drinking patterns, alcohol-related attitudes and alcohol-related risk behaviors. Data is presented in tabular format. A table showing differences among survey data sources is appended. (Contains 33 figures and 15 tables.)