The relationship between accidents and number of children in the household was assessed in 10,394 children surveyed at ages 5 and 10 years. The analyses suggest that living in a household with 3 or more children during the preschool period increases a child's risk of experiencing accidents that result in hospitalization; and that living in a household with 4 or more children increases the risk of such accidents to school-age children. The number of older rather than younger children had the greatest impact on accident risk. The observed odds ratios suggest that children with 4 or more siblings have 80% to 90% more injuries resulting in hospitalization than only children. The proportions of children with one or more accidents (regardless of the place of treatment) and with repeat accidents were unrelated to family size. Environmental differences between families of varying size accounted for the association with hospitalized accidents.