BACKGROUND The observed activity of cisplatin in breast cancer and its unattractive toxicity profile in palliative treatment warranted further study of platinum analogues in this disease. METHODS Sixty-two patients with recurrent or metastatic breast cancer, 61 of whom had been previously treated with chemotherapy, were randomly assigned to therapy with either iproplatin (n = 32) or carboplatin (n = 30). Both platinum analogues were administered intravenously, iproplatin at a dose of 240 mg/m2 every 4 weeks and carboplatin at a dose of 450 mg/m2 every 5 weeks. RESULTS Only two patients responded to iproplatin (7%) for durations of 21 and 61 weeks, and one patient responded to carboplatin (3%) for a duration of 64 weeks. All responses were complete. At the given dose schedules carboplatin was more myelosuppressive than iproplatin. Non-hematologic toxicities included nausea and vomiting (93% vs. 90%), diarrhea (20% vs. 10%) and hemorrhage (16% vs. 10%) for iproplatin and carboplatin, respectively. Two patients developed alopecia with carboplatin. No renal toxicity was observed. CONCLUSIONS Both iproplatin and carboplatin have limited activity in previously treated women with advanced breast cancer when given in conventional dosages.