The National Registry of Childhood Tumours contains over 51000 records of children born in Great Britain who developed cancer under the age of 15 years. Patterns of childhood cancer among families containing more than one child with cancer have been studied. A total of 225 "sib pair' families have been ascertained from interviews with parents of affected children, from hospital and general practitioner records and from manual and computer searches of names and addresses of patients. A number of special groups have been identified, including those with a known genetic aetiology such as retinoblastoma, twins and families with three or more affected children. A further 148 families not in any of the above groups contain two children with cancer: in 46 families the children had tumours of the same type, most commonly leukaemia. Some of the families are examples of the Li-Fraumeni syndrome; some are associated with other conditions, including Down's syndrome. There is clearly a genetic element in the aetiology of cancer in some families discussed here; shared exposure to environmental causes may account for others and some will be simply due to chance.