In a phase II study, 35 patients with advanced breast cancer were treated with 4'-O-tetrahydropyranyl-doxorubicin (THP-DXR) (70 mg/m2 i.v. on day 1); treatment was repeated every 3 weeks. Eight patients had failed prior chemotherapy for advanced disease. A total of 34 patients were evaluable for response. After a median of 10 treatment courses (range, 3-15), objective tumor response was seen in 59% (20 of 34 patients) (95% confidence limits, 42%-75%). In all, 17 partial remissions and 3 complete remissions were observed; stable disease occurred in 13 patients. The median duration of response was 42+ weeks (range, 21 - 77+ weeks). The dose-limiting side effects were leukopenia (26 patients, WHO grade III-IV) and thrombocytopenia (9 patients, WHO grade II-IV). Nausea/vomiting was experienced by 34 patients; in 18, it reached WHO grade II-III. Other treatment-related side effects included alopecia (WHO grade II-III) in 26 patients and stomatitis and diarrhea (WHO grade I-III) in 9 patients. At cumulative doses of THP-DXR of at least 700 mg/m2 (range, 700-1,050 mg/m2), no signs of congestive heart failure were observed. We conclude that THP-DXR is effective for first- and second-line chemotherapy in advanced breast cancer and that side effects are manageable.