Forty-six eligible women with advanced metastatic breast cancer were entered on a Phase II trial utilizing esorubicin (4'-deoxydoxorubicin) given in a dosage of 30 mg/m2 intravenously every 3 weeks. No patient had received anthracyclines or cytotoxic therapy for metastatic disease. Twenty-three (50% of patients) had prior adjuvant chemotherapy, 21 (46%) had prior hormonal therapy, and 32 (70%) were postmenopausal. Dominant site of disease was visceral in 26 (57%), bone in 14 (30%), and soft tissue in 6 (13%). There were 3 complete and 13 partial responders observed, for a 35% response rate; 95% confidence interval for response was 21-49%. Median response duration was 4.0 months (range 2-21 months), and one partial responder remains on study at 6.3 months. Thirty-nine of 46 patients have died; median survival was 10.1 months. Toxicity was primarily hematologic, with 2 drug-related septic deaths. In addition, 2 patients developed severe congestive heart failure secondary to esorubicin cardiotoxicity (at 687 and 770 mg/m2, respectively), which resulted in one patient death. Nausea and vomiting were severe in 16% of patients, but total alopecia was only noted in 4 (9%). Esorubicin is an active agent in metastatic breast cancer; its role in treatment remains undefined.