The purpose of this study was to determine the maximal tolerated dose of thiotepa administered with busulfan 12 mg/kg and melphalan 100 mg/m2 followed by autologous stem cell transplantation in patients with refractory malignancies. Twenty-eight patients with refractory malignancies received high-dose busulfan 12 mg/kg, melphalan 100 mg/m2 and escalating doses of thiotepa 450-550 mg/m2 followed by infusion of cryopreserved autologous peripheral blood stem cells (n = 26) or marrow (n = 2). The maximum tolerated dose was determined to be busulfan 12 mg/kg, melphalan 100 mg/m2 and thiotepa 500 mg/m2. Two of three patients receiving thiotepa 550 mg/m2 experienced grade 3 colitis. Twenty patients were enrolled at the maximum tolerated dose and the incidence of grade 3-4 regimen-related toxicity and mortality was 10% and 5%, respectively. Ninety-five per cent of patients experienced grade 1-2 mucositis, 50% grade 1-2 gastrointestinal toxicity, 35% grade I hepatic toxicity and 20% experienced grade 1-2 skin toxicity. The median time to achieve a granulocyte count of 0.5 x 10(9)/I was 10 days (range 8-20 days) and platelet transfusion independence was 10 days (range 1-26 days). Five of ten patients with stage 4 refractory breast cancer achieved a complete and two a partial remission with a complete response rate of 50% and a overall response rate of 70%. In conclusion, busulfan, melphalan and thiotepa can be administered in high doses with tolerable mucositis as the major side-effect. This combination has significant activity in patients with breast cancer, and phase II studies in patients with breast cancer and other chemotherapy sensitive malignancies are warranted.