Early life events have important short- and long-term consequences. It is clear from previous studies that birthweight is associated with infant mortality and with childhood and adult morbidities. However, few studies have focused on the relationship between birthweight and childhood mortality. To assess this relationship, we conducted a population-based case–control study of children born during 1968–96 in Washington state. Cases consisted of 6247 children who died at 1–19 years of age. A total of 31 074 controls were matched five to one to cases by birth year. Compared with children with a birthweight of 3000–3499 g, children with lower birthweights had a greater risk of childhood mortality. These lower birthweight children had increased risks of childhood deaths from infectious diseases, congenital anomalies, central nervous system diseases and heart disease, but not of deaths resulting from accidents, cancer, suicide or homicide. The magnitude of these risks differed somewhat by age. Our results suggest that birthweight exerts important influences on children's risk of age-specific and cause-specific mortalities, particularly those with a strong biological component.