Twenty-eight patients with advanced breast cancer refractory to prior hormone and/or first-line chemotherapy (with or without anthracycline drugs) were treated with the investigational agent amonafide at a dose of 800 mg/m2 intravenously over 3 hours repeated every 4 weeks. Five objective tumour responses of 5.0 months' median duration were observed in the 20 patients without previous anthracycline exposure, including 1 CR. Leukopenia was the dose-limiting toxicity; though it was generally modest with the 800 mg/m2 amonafide starting dose, an initial dose reduction should be considered in patients with prior radiotherapy and/or bone marrow involvement. Other adverse reactions included nausea/vomiting (53%), phlebitis/erythema along the vein injected (7%), and mild neurotoxic symptoms during the drug administration such as headache, tinnitus, and diaphoresis (21%). Amonafide is an active compound for the treatment of patients with advanced breast cancer and should be considered for further evaluation and incorporation in combination chemotherapy.