It is well recognized that serious illness in children can have important consequences for siblings who may manifest their concerns as behavioural difficulty. The aim of the present study was to explore sibling psychological response to childhood cancer and to observe the effects that sibling knowledge had on their response. Siblings and parents of patients at the Children's Hospital, Birmingham, were interviewed using the Achenbach Child Behavior Checklist and semi-structured 'Illness Knowledge' questionnaires designed by the first author. Siblings were not experiencing significant behaviour problems. The level of sibling knowledge about cancer was related to the level of sibling social competence but not to the prevalence of behaviour problems. One-third of the siblings were talking to no one at all about their feelings, and sibling feedback yielded some important sibling needs. It was concluded that sibling response to childhood cancer may not necessarily be pathological and may also be mediated by the level of sibling social competence as well as sibling knowledge.