The development of sex differences in children's injury rates was explored by analyzing data from 197,516 consumer product-related injuries reported in 1978. The results indicated that sex differences in injury rates appear within the first year of life for most types of injuries. Burns, ingestions, and poisonings were important exceptions. The sex differences were not completely explained by differences in exposure to risk. Sex differences in behavior begin to appear at the same age as the differences in injury rates and correlate with injury type. Injury prevention efforts should take these developmental differences into account and focus attention on the high-risk child.