We have reviewed the records of the 16,564 cases of childhood cancer diagnosed from 1971 to 1983 which were reported to the National Registry of Childhood Tumours in Great Britain for the presence of underlying genetic disease in order to estimate the proportion which results from inherited mutations. A genetic condition was listed for 509 patients, or 3.07% of the total number of tumours. The most frequently recorded diagnoses were: bilateral retinoblastoma (162 cases); Down syndrome (135); neurofibromatosis (90); hereditary Wilms' tumour (71); and tuberous sclerosis (20). The highest hereditary fractions at individual tumour sites were seen for: retinoblastoma (37.2%); kidney (7.2%); leukaemia (2.6%) and brain and spinal cord (2.0%). When information about family history from published reports was incorporated into the figures calculated from Registry data the total genetic fraction was estimated to be 4.2%. We conclude that there is a clear genetic basis for a small minority of the cancers of childhood, but ethnic variation and the lack of known environmental determinants suggest that the total influence of heredity may be higher.