The purpose of this study was to determine the toxicities and potential effectiveness of high-dose busulfan, melphalan and thiotepa (Bu/Mel/TT) followed by autologous peripheral blood stem cell (PBSC) infusion in patients with a variety of diseases. A phase II clinical trial of Bu (12 mg/kg), Mel (100 mg/m2) and TT (500 mg/m2) followed by PBSC infusion in 104 patients with breast cancer (n = 48), malignant lymphoma (n = 25), ovarian cancer (n = 13), multiple myeloma (n = 7) and other malignancies (n = 11) was performed. Sixty-two patients were treated in an academic medical center and 42 in a community cancer center. Grade 3-4 regimen-related toxicities occurred in 14% of patients, causing regimen-related mortality in six (6%) patients with an overall transplant-related mortality of 9%. Transplant-related deaths occurred in 6/62 patients (10%) treated in an academic medical center and in 3/42 (7%) treated in a community cancer center. Complete remissions (CR) were achieved in 1/17 (6%) patients with refractory stage IV breast cancer, 4/4 patients with responsive stage IV breast cancer, 6/13 (46%) with more-advanced lymphoma and 4/4 with less-advanced lymphoma. These patients are alive and disease-free a median of 712, 279, 461 and 404 days after transplant, respectively. Nineteen of 22 patients with stage II-III breast cancer remain alive and disease-free a median of 365 days after transplant. Complete remissions were also seen in 4/9 patients with ovarian cancer and 3/7 with multiple myeloma. The Bu/Mel/TT regimen followed by autologous PBSC infusion is associated with acceptable morbidity and mortality, appears to have significant activity in patients with breast cancer and is well tolerated in the adjuvant setting of stage II-III breast cancer. Bu/Mel/TT also appears to have significant activity in patients with lymphoma, multiple myeloma and possibly ovarian cancer. Further phase II-III studies are warranted in patients with these and other malignancies.