Thirty-two patients with advanced breast cancer were treated with mitomycin-C 10 mg/m2 IV and vinblastine 6 mg/m2 IV every 21 days in combination with lonidamine 450 mg/day P.O. and prednisone 15 mg/day P.O. given continuously. Among the 29 evaluable patients (all but three pretreated with an anthracycline-based regimen), one complete remission (CR) and six partial remissions (PR) (response rate, 24%) were seen. The median duration of response was 14 months (range, 4-30 months). Median survival for responders was 18 months (range, 4-30 months). Hematological toxicity was uncommon; the main lonidamine-related side effects were myalgia, abdominal cramps, and reversible deafness; these side effects were severe in two, one, and one patients, respectively. The regimen seems to have a reasonable degree of activity and toxicity in advanced, refractory breast cancer. The role of lonidamine in the treatment of this disease warrants further evaluation.