Single agent carboplatin has demonstrated antitumoral activity in patients with advanced breast cancer. Thirty patients with inoperable locally advanced and/or metastatic breast cancer were treated with carboplatin (300 mg/m2) in combination with cyclophosphamide (600 mg/m2) and 5-fluorouracil (500 mg/m2) given on day 1 in a 4-weekly schedule. Of 29 patients evaluable for response, 4 presented CR and 4 PR (28%). Seven out of 19 chemotherapy-naive patients achieved CR (4) or PR (3) (37%). In contrast, only one patient out of 10 achieved PR in the group with previous adjuvant chemotherapy (10%). Responses were observed in primary tumours as well as in metastatic sites, including lymph nodes, lung, liver and skin. Median duration of response was 7.5+ and 3.8 months in CR and PR patients respectively. Toxicity was generally mild. Only 2 patients presented with clinically relevant hematologic toxicity. No significant non-hematologic toxicity was observed. It appears that this regimen, at the dosage and schedule studied, possesses only modest activity in patients with breast cancer, while being relatively atoxic. Carboplatin merits further investigation in this disease, but dosing should be individualised using e.g. a pharmacokinetic formula.