Thirty-one patients with metastatic breast cancer not responding or progressing after initial response to adriamycin + cyclophosphamide (AC) treatment entered a phase 11 study with oral lonidamine in association to AC. Objective clinical responses were observed in 10 patients (32%) and consisted of 1 complete + 9 partial remissions. Disease stability and progression were observed in 8 and 13 cases, respectively. These results were obtained with a marginal toxicity in addition to that already reported for AC therapy, the main additional side effect being myalgia, which was easily manageable in most cases.