Objective: The authors examined drinking and driving after drinking before and after turning 21. Participants: Participants were drawn from first time college students who were taking part in a 4-year longitudinal study of alcohol use and behavioral risks. Methods: Web-based longitudinal surveys collected data on drinking and driving after drinking from August 2004 through November 2007 (n = 1,817). A subset of participants (n = 224) also monitored their daily behavior during the month they turned 21 (January through May, 2007). Results: Typical frequency and quantity of alcohol use increased from ages 18 to 21 years, whereas quantity decreased between 21 and 23 years of age. Driving after drinking showed a 72% relative increase (6% absolute increase) in the 2 weeks after turning 21. Conclusions: Reaching the legal drinking age is associated with decreases in the amount of alcohol consumed per drinking occasion, but an increase in driving after drinking. (Contains 1 table and 4 figures.)