High-dose chemotherapy with hematopoietic progenitor cell support is administered increasingly to selected categories of patients with high-risk malignancies. Bone marrow and/or peripheral blood progenitor cells (PBPCs) are commonly cryopreserved with the cryoprotectant dimethyl sulfoxide (DMSO), which can cause a variety of systemic side effects when the graft is thawed and infused. The progenitor cells thought to be responsible for hematopoietic recovery express the CD34 antigen and constitute 1% to 3% of the marrow cells and 0.5% of the PBPC fraction. Transplantation of a CD34(+) graft would markedly reduce the volume and thus the amount of DMSO required, thereby decreasing the infusion-related toxicities. In this study, 89 high-risk breast cancer patients received high-dose therapy and were randomized to receive an autologous CD34(+) marrow graft (Arm A) versus a standard buffy coat fraction (Arm B). After marrow infusion, significant increases in diastolic and systolic blood pressure, as well as significant decreases in heart rate, were documented in Arm B compared to Arm A patients (P < .001). None of the patients in Arm A experienced any clinically serious adverse events associated with the marrow infusion compared to 6% of the Arm B patients. The median time to neutrophil engraftment was 13 days for Arm A and 11 days for Arm B patients (P = .218). The median time to platelet engraftment was 27 days for Arm A and 20 days for Arm B patients (0.051). There were no other significant differences between the two arms of the study with respect to thrombocytopenia-related complications or immune function reconstitution. Additionally, patients on Arm A who received >/=1.2 x 10(6) CD34(+) cells/kg had no delay in platelet recovery (22 days), compared to patients on Arm B, who also received greater than 1.2 x 10(6) CD34(+) cells/kg (20 days) (P = .604). In conclusion, this prospective randomized study demonstrates that breast cancer patients who receive high-dose therapy with autologous CD34(+) marrow support have reduced marrow infusion-related toxicity, comparable time to neutrophil engraftment and immune function recovery posttransplant, and for those who receive <1.2 x 10(6) CD34(+) cells/kg, comparable time to platelet engraftment compared to women who receive buffy coat fractions of marrow.