A non-randomized study was carried out in the Free University Hospital, Amsterdam, to investigate the (hematologic) toxicity and antitumor response of patients with advanced breast cancer treated with intensive chemotherapy in combination with granulocyte-macrophage colony-stimulating factor (GM-CSF). Of 11 patients with an inoperable or metastasized breast cancer, 5 were treated with doxorubicin 75 mg/m2 + cyclophosphamide 750 mg/m2 intravenously every 3 weeks and 6 patients with 90 and 1000 mg/m2 respectively. When in a preceding cycle a significant hematologic toxicity was observed, this patient was treated in the subsequent cycle with the same dose of chemotherapy in combination with GM-CSF 250 micrograms/m2/day from day 2-12 as a continuous infusion. Bone marrow depression was diminished in the presence of GM-CSF. This was apparent from a milder decline of the number of neutrophilic granulocytes, reduction of the neutropenic period and a more rapid recovery of the neutrophil number. A transient eosinophilia and a mild monocytosis were also observed. GM-CSF did not improve erythrocyte and thrombocyte counts. The efficacy of GM-CSF was less pronounced in the group of patients with the highest dose of chemotherapy. GM-CSF was associated with malaise, fever and a small decrease of blood pressure, which in combination with a frequently occurring anemia and the side-effects of high dose chemotherapy, resulted in a substantial toxicity. In 9/11 patients an objective tumor regression was noted. GM-CSF stimulated the recovery of granulocytes after intensive chemotherapy. Treatment of a small group of patients with advanced breast cancer with intensive chemotherapy resulted in a high antitumor response.