The aim of this study was to determine if lonidamine (LND) supplementation to single-agent epirubicin (EPI) could reverse anthracycline resistance in patients with metastatic breast cancer. 45 patients with metastatic breast cancer were treated with EPI 120 mg/m2 by intravenous (i.v.) bolus every 3 weeks. Patients who progressed were given the same chemotherapy regimen on day 4 in combination with oral LND, 150 mg on day 1, 300 mg on day 2 and 450 mg on days 3-5. Among the 40 evaluable patients, 6 complete responses (CR) and 14 partial responses (PR) were achieved with EPI treatment alone for an overall response rate of 50%. The median duration of response was 6.5 months. Among the 25 patients treated with EPI+LND, 5 PR (21% of 24 evaluable patients) were observed with a median duration of response of 7 months. The median survival in patients receiving both treatments was 20 months. The survival for all patients was 18 months. The survival of patients receiving LND was not significantly longer than for the other patients. Myelotoxicity was the most common side-effect followed by alopecia, nausea and vomiting, and stomatitis. LND-related toxic effects were mild-to-moderate epigastralgia and myalgia. Anthracycline-related toxicity was the same in the two treatment groups. This study indicates that LND may circumvent clinical resistance to EPI without altering the pattern or severity of the toxicity of this anthracycline. Continued investigation of the clinical modulation of EPI resistance by LND in breast cancer is warranted, hopefully in patients with known multidrug resistance status.