In an effort to evaluate the toxicities and anti-tumor efficacy of the combination of high-dose cyclophosphamide (CY) and carboplatin, we undertook a phase I-II trial with autologous bone marrow (BM) or peripheral blood stem cell (PBSC) rescue for patients with solid tumors. Forty three patients, 39 of whom had either high risk stage II or III or metastatic breast cancer were treated with escalating doses of carboplatin 1200-1800 mg/m2 and cyclophosphamide 4800-6000 mg/m2 over 3 days followed by autologous BM or PBSC infusion. No life-threatening or fatal toxicities were observed. Reversible congestive heart failure was seen in two patients. Transient hepatotoxicity, characterized primarily by elevation of transaminase levels, and nausea and vomiting, adequately managed with anti-emetic therapy, were seen in 39 and 40 of 43 patients, respectively. The 14 month post-transplant probability of relapse-free survival for 26 patients with high risk II-III breast cancer was 79%; for 13 patients with metastatic disease, the 22 month relapse-free survival probability was 23%. High-dose carboplatin and CY at maximally administered doses of 1800 mg/m2 and 6000 mg/m2 is a well tolerated preparative transplant regimen for autologous BM or PBSC transplantation. It appears to have similar anti-tumor activity and an improved safety profile when compared with other commonly employed transplant preparative regimens.