Participants Assessment of risk factors Children were considered to be exposed for 90 days after ?1 of their siblings were admitted to hospital or treated in the ED for an injury. Children were considered to be unexposed if there were no recorded injuries for siblings or if &gt;90 days had elapsed since an injury. Main outcome measures Main results 5003 incident injury events occurred in 4162 children; 4921 events required treatment in the ED (4084 children) and 82 events required admission to hospital (78 children). Children who had a sibling who had been injured in the previous 90 days had an increased risk of injury treated in the ED (hazard ratio 1.53, 95% CI 1.34 to 1.74, adjusted for age, sex, race, sibling group size, and sibling group ED use for non-injury diagnoses), but not for injury requiring hospital admission (adjusted hazard ratio 0.46, CI 0.13 to 1.64). The risk of injury after a medically treated injury in a sibling was highest in the second week after the event (adjusted hazard ratio 2.24, {CI 1.66 to 3.01}*); risk was raised &gt;50% above baseline for 30 days after the event and declined toward the baseline risk by 90 days. Data analysed separately by age group showed that exposure to an injury in a sibling was protective in children &lt;12 months of age; for all other age groups, exposure was associated with an increased risk, and the magnitude of this risk increased with age. Conclusions Siblings of children treated at the emergency