Context: Although more than a dozen states have ratified laws that require booster seats for children older than 4 years, most states continue to have child restraint laws that only cover children through age 4 years. Lack of booster seat effectiveness data may be a barrier to passage of stronger child restraint laws.

Objectives: To quantify the association of belt-positioning booster seats compared with seat belts alone and risk of injury among 4- to 7-year-old children and to assess patterns of injury among children in booster seats vs seat belts.

Design, setting, and population: Cross-sectional study of children aged 4 to 7 years in crashes of insured vehicles in 15 states, with data collected via insurance claims records and a telephone survey. A probability sample of 3616 crashes involving 4243 children, weighted to represent 56 593 children in 48 257 crashes was collected between December 1, 1998, and May 31, 2002.

Main outcome measure: Parent report of clinically significant injuries.

Results: Injuries occurred among 1.81% of all 4- to 7-year-olds, including 1.95% of those in seat belts and 0.77% of those in belt-positioning booster seats. The odds of injury, adjusting for child, driver, crash, and vehicle characteristics, were 59% lower for children aged 4 to 7 years in belt-positioning boosters than in seat belts (odds ratio, 0.41; 95% confidence interval, 0.20-0.86). Children in belt-positioning booster seats had no injuries to the abdomen, neck/spine/back, or lower extremities, while children in seat belts alone had injuries to all body regions.

Conclusion: Belt-positioning booster seats were associated with added safety benefits compared with seat belts to children through age 7 years, including reduction of injuries classically associated with improper seat belt fit in children.