The National Cancer Institute of Canada (NCIC) Clinical Trials Group conducted a phase II study of the oral antiandrogen flutamide in 33 patients with metastatic breast cancer. Eight patients had received no prior systemic therapy for their metastatic disease and 13 had only one site of metastasis. Toxicity occurred in 18 of the 33 patients and was primarily gastrointestinal. It ranged in severity from mild to severe with 4 patients discontinuing treatment early because of nausea, vomiting, diarrhea or stomatitis. One response, of 8 weeks duration, was noted in 29 evaluable patients. We conclude that flutamide does not have meaningful antitumour activity in breast cancer and plan no further trials of the drug in this disease.