One hundred and sixty one children who have developed more than one primary neoplasm have been identified. Children with tumours of the central nervous system, retinoblastoma and leukaemia were those most frequently observed to develop a second malignancy whilst osteosarcoma was the most common second tumour. The patterns of second neoplasms appear to be changing and a recent increase in the number of children with leukaemia and lymphoma who develop second primary tumours has been observed. In this series, the two most frequent associations of tumours were retinoblastoma followed by osteosarcoma and the combination of acute leukaemia with a tumour of the central nervous system. Genetic factors which may have contributed to the development of the second primary tumour were identified in 53 patients (33%), 33 of whom had the genetic form of retinoblastoma. In an analysis of the treatment of 151 patients, for whom the interval between the two neoplasms was greater than 12 months, the second malignancy was considered to be 'radiation associated' in 93 (61%). Fifty children (33%) had been treated with either single or multiple agent chemotherapy which included an alkylating agent in 38. Forty five children had received a combination of chemotherapy and radiotherapy and of these, 10 developed leukaemia as their second tumour. Of the 19 secondary leukaemias, 16 have occurred in patients treated since 1970.