To compare child care center-based booster seat education and distribution with no intervention when implemented immediately after booster seat legislation. Design: Cluster randomized controlled trial. Setting: Thirty-nine urban child care centers. Participants: Eight hundred fifty-four parents and 1010 children aged 4 to 8 years in vehicles leaving centers. Intervention: We trained 168 staff members at 20 centers to give parents and children messages promoting booster seats and supplied lesson plans, children's activities, and free booster seats. Main Outcome Measures: Observed booster seat use, "good practice" restraint use, and legal restraint use. Results: Parents at intervention centers were more likely to report receiving restraint information from the center (adjusted odds ratio [AOR], 4.06; 95% confidence interval [CI], 2.48-6.67), speaking with staff about booster seats (AOR, 3.95; 95% CI, 2.26-6.88), and using fit to decide when to move children into seat belts (AOR, 3.39; 95% CI, 1.91-5.99). Groups did not differ in proportions using booster seats (44% vs 43%; AOR, 1.03; 95% CI, 0.62-1.73), good practice (42% vs 41%; AOR, 1.11; 95% CI, 0.70-1.74), or legal restraints (65% vs 65%; AOR, 0.79; 95% CI, 0.48-1.31). Results were similar for children aged 4 to 5 and 6 to 8 years. All outcomes were significantly less likely among children riding in pickup trucks or with Hispanic or black drivers. Conclusions: The intervention increased parents' receipt of information from center staff and knowledge about booster seats but not booster seat use. Research is needed to identify methods and messages that will empower center providers to promote booster seats effectively and reach high-risk populations.