To evaluate the effect of high-dose chemotherapy in the treatment of metastatic breast cancer, we performed a phase II trial of a single treatment with high-dose cyclophosphamide (5,625 mg/m2), cisplatin (165 mg/m2), and carmustine (600 mg/m2), or melphalan (40 mg/m2) and bone marrow support as the initial chemotherapy for metastatic breast cancer. Twenty-two premenopausal patients with estrogen receptor negative, measurable metastatic disease were treated. Twelve of 22 patients (54%) obtained a complete response at a median 18 days. The overall response rate is 73% (complete and partial response). Median duration of response in the patients achieving complete response was 9.0 months with a median duration of survival for complete responders that is currently undefined. Relapse occurred predominantly at sites of pretreatment bulk disease or within areas of previous radiation therapy. Toxicity was frequent and five patients died of therapy-related complications. The results indicate that a single treatment with intensive combination alkylating agents with bone marrow support can produce more rapid and frequent complete responses than conventional chemotherapy when used as initial chemotherapy for metastatic breast cancer, although median disease-free and overall survival is not improved. Three patients (14%) remain in unmaintained remission beyond 16 months.