The objective of this study was to determine the effectiveness of forward facing child restraint systems (FFCRS) in preventing serious injury and hospitalization to children 12-47 months of age as compared with similar age children in seat belts. Data were obtained from a cross-sectional study of children aged 12-47 months in crashes of insured vehicles in 15 states, with data collected via insurance claims records and a telephone survey. Effectiveness estimates were limited to those children between 12 and 47 months of age seated in the back row(s) of vehicles, restrained in FFCRS, regardless of misuse, or seat belts of all types and usage. Completed survey information was obtained on 1207 children, representing 12632 children in 11619 crashes between 1 December 1998 and 31 May 2002. Serious injuries occurred to 0.47% of all 12-47-month olds studied, including 1.72% of those in seat belts and 0.39% of those in child restraint systems. The risk of serious injury was 78% lower for children in FFCRS than in seat belts (odds ratio (OR) = 0.22, 95% confidence interval (CI) = 0.11-0.45, P = 0.001). The risk of hospitalization was 79% lower for children in FFCRS than in seat belts (OR = 0.21, 95% CI = 0.09-050, P = 0.001). There was no difference between the restraint types in preventing minor injuries. As compared with seat belts, CRS are very highly effective in preventing serious injuries and hospitalization, respectively. This effectiveness estimate is substantially higher than older estimates, demonstrating the benefits of current CRS designs. These results provide those educating parents and caregivers population-based data on the importance of child restraint use.