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1 Objectives: We evaluated the effects of lowered allowable blood alcohol concentration (BAC) limits for drivers under age 21 in 30 U.S. states between 1984 and 1997. Outcome measures included drinking, heavy episodic drinking, driving after drinking, driving after heavy drinking, riding with a drinking driver, riding with a heavy drinking driver, and total exposure measured by miles driven. Methods: The design involved repeated cross-sectional probability-sample surveys of high school seniors in 30 states. Outcomes measures were based on self-reports from samples of over 5000 high school seniors before and after the laws took effect. Results: Frequency of driving after any drinking and driving after five or more drinks declined 19 % and 23%, respectively, after lower youth BAC limits were implemented. The size of the effects in terms of percentage of baseline standard deviation were-6.4 and-5.7. Effects remained after controlling for broader secular trends. 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. BAC Limits for Young Drivers