We evaluated 9-aminocamptothecin (9-AC) in patients with metastatic or locally recurrent breast cancer who were no longer responsive to standard therapy. Patients were treated with 9-AC with a 72-hr continuous infusion given at a dose of 45 micrograms/m2/hr every 2 weeks. Granulocyte colony-stimulating factor 5 micrograms/kg was given subcutaneously for 7-10 days after completion of the treatment. Eighteen patients were treated, with all patients assessable for toxicity and 15 patients assessable for response. There were two partial responses seen in the 15 patients lasting 3.5 and 5 months, respectively. The major toxicity seen was myelosuppression, with 12 patients having grade 3 or greater granulocytopenia with four episodes of significant infectious complications. In addition, significant thrombocytopenia was seen in 14 patients. The other complications commonly seen were nausea and vomiting and alopecia. 9-AC given as a 3-day continuous infusion has limited activity in previously treated metastatic and locally recurrent breast cancer.