Fifty children were induced with busulphan and cyclophosphamide before bone marrow transplantation. Our standard dose of cyclophosphamide was 2 g/m2 daily for 4 days, but this dosage was reduced if necessary so as not to exceed 75 mg/kg/day. A basal pulse rate that increased by more than 20 beats/min or a reduction in ECG voltage were indications for stopping cyclophosphamide. As a consequence, the fourth dose of cyclophosphamide was omitted for four children. One child died of cardiac arrest related to cyclophosphamide. We used busulphan at a dosage of 4 mg/kg/day for 4 days; neither 'busulphan lung' nor veno-occlusive disease occurred in any patient. The use of busulphan and cyclophosphamide did not guarantee freedom from interstitial pneumonitis, but this seemed to be related rather to the nature of prior treatment and the cytomegalovirus antibody status of the patient. Thus, we have developed a safe chemotherapy schedule for transplantation that avoids the need for radiotherapy and can, if necessary, be repeated after a 3-month interval.