(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.