The toxicity of the conditioning regimen high-dose busulfan (BU) 16 mg/kg followed by cyclophosphamide (CY) 200 mg/kg has been analysed in 60 adult patients (mean age 36 +/- 9 years) with haematological malignancies, a third of whom had advanced disease, all received the graft from fully HLA-identical siblings. Significant nausea and vomiting were rare during BU administration but occurred in 44% of the patients with CY. Severe mucositis occurred in 30% of patients. Haemorrhagic cystitis occurred in 16% of patients; interstitial pneumonia occurred in 3 patients and was fatal in one. Veno-occlusive disease of the liver occurred in 2 patients and was fatal in one: however, increase of bilirubin of at least twice the baseline value and/or isolated weight gain > 5% of pre-transplant value occurred in 28% of patients. These signs of liver toxicity disappeared in all patients after appropriate therapy. Normalisation of bilirubin levels took twice as long as normalisation of body weight: median 35 and 18 days, respectively. Hyperpigmentation of the skin, mainly involving flexural and pressure areas, occurred in 47% of patients and was manageable topically. Eight patients died of relapsed disease; 15 died of transplant complications but in six the original malignancy persisted or had recurred at the time of death. Overall transplant-related mortality was 15%. We conclude that the toxicity of this regimen has not been high, with the liver being the most seriously affected organ. A longer follow-up is necessary to assess long-term consequences.