The authors analyzed patterns of criminal and administrative enforcement of the legal minimum age for drinking across 295 counties in four States. Data on all arrests and other actions for liquor law violations from 1988 through 1990 were collected from the Federal Bureau of Investigation Uniform Crime Reporting System, State Uniform Crime Reports, and State Alcohol Beverage Control Agencies. Analytic methods used include Spearman rank-order correlation, single-linkage cluster analysis, and multiple regression modeling. Results confirmed low rates of enforcement of the legal drinking age, particularly for actions against those who sell or provide alcohol to underage youth. More than a quarter of all counties examined had no Alcoholic Beverage Control Agency actions against retailers for sales of alcohol to minors during the three periods studied. Analyses indicate that 58 percent of the county-by-county variance in enforcement of the youth liquor law can be accounted by eight community characteristics. Rate of arrests for general minor crime was strongly related to rate of arrests for violations of the youth liquor law, while the number of law enforcement officers per population was not related to arrests for underage drinking. Raising the legal age for drinking to 21 years had substantial benefits in terms of reduced drinking and reduced automobile crashes among youths, despite low level of enforcement. Potential benefits of active enforcement of minimum drinking age statutes are substantial, particularly if efforts are focused on those who provide alcohol to youth.