The problem of second primary tumours is likely to increase in magnitude as the number of long-term survivors of childhood cancer grows and treatment protocols are intensified. Children with an underlying genetic disease appear to be at particular risk of developing a second tumour. While most cases of second malignancy appear to be associated with either radiotherapy or chemotherapy, a small proportion of patients have no identifiable risk factor. In these children, unrecognised predisposition or, indeed, chance may play a role. It would seem to be important to identify factors such as genetic susceptibility and specific modalities of therapy, including ionising radiation and alkylating agents, which may contribute to the development of second tumours, because awareness of the risk factors may make it possible to modify treatment programmes and thereby minimise the risk of second neoplasms. Long-term surveillance of patients treated for cancer during childhood is recommended so that the problem of second malignancies can be monitored.