The aim of this study was to determine the therapy-related risk factors for the occurrence of leukaemia after childhood solid cancer. Among 4204 3-year survivors of a childhood cancer treated in eight French and British centres before 1986, 11 patients developed leukaemia as a second malignant neoplasm (SMN). Compared with the leukaemia incidence in the general French and British populations, the standardised incidence ratio (SIR) of leukaemia was 7.8 (95% CI 4.0-13.4). It decreased from 20.3 (95% CI 8.3-41.2) during the first years of follow-up, to 2.2 (95% CI 0.1-9.7) between 10 and 20 years, but rose again to 14.8 (95% CI 3.7-38.3) 20 or more years after the first cancer. Radiotherapy appeared to increase the risk of leukaemia at moderate weighted doses to active bone marrow; the relative risk (RR) was 4.2 (95% CI 0.8-20.7) for doses ranging from 3 to 6.6 Gy. A greater RR was observed for epipodophyllotoxins and for vinca alkaloids. No specific type of first malignant neoplasm (FMN) was found to lead to a higher risk of secondary leukaemia. Epipodophyllotoxins and vinca alkaloids at high doses and moderate weighted radiation doses to active bone marrow may contribute independently to an increased risk of leukaemia for patients treated for childhood cancer. Our results suggest that the long-term risk of secondary leukaemia could be higher than previously reported.