The changes occurring within Korean society provide an opportunity for studying the influence of family structure on children's emotional and behavioural problems. Children aged 7–13 years from two Korean cities were assessed for emotional and behavioural problems in school by their teachers, using the Children's Behaviour Questionnaire. In Study 1, 326 children from extended families were compared with demographically matched nuclear family children in the same school classes. In Study 2, a further sample of 204 extended family children was compared with pair-matched nuclear family children, in order to replicate the findings. Children from Study 1 were followed up 2.5 years later. Children from extended families had lower behaviour problems scores and the prevalence of serious problems was lower in extended family children. These differences were most marked in relation to externalising behaviour problems and were stable over the studies and time. Grandparents in extended families may increase children's resiliency by providing sources of attachment, affection, and knowledge, as well as having indirect effects through their support of parents. Consistent with recent ideas about the cognitive bases for behaviour problems, it may be that rules for behaviour derived from traditional cultural beliefs and values are internalised by children from extended families and generalise to prevent behaviour problems in school.