OBJECTIVES: This study evaluated the effects on drinking and driving of lowered allowable blood alcohol concentration (BAC) limits for drivers younger than 21 years in 30 US states between 1984 and 1998. METHODS: Outcome measures were based on self-reports from a cross-sectional sample of more than 5000 high school seniors in 30 states surveyed before and after BAC limits were implemented in their states. RESULTS: Frequency of driving after any drinking and driving after 5 or more drinks declined 19% and 23%, respectively. Lower BAC limits did not affect overall amount of drinking or total number of miles driven. CONCLUSIONS: Significant beneficial effects of lowered youth BAC limits have appeared despite limited publicity and enforcement of the new laws.