Phase II studies have demonstrated that docetaxel (Taxotere; Rhône-Poulenc Rorer, Collegeville, PA) is one of the most active single agents in the treatment of metastatic breast cancer. The overall response rate as front-line therapy for metastatic disease was 59% (95% confidence interval, 51% to 67%) in five phase II trials (four of which were multicenter) when 100 mg/m2 docetaxel was infused over 1 hour every 3 weeks. In the three phase II trials reported to date of patients with metastatic cancer who had failed previous frontline therapy, 100 mg/m2 docetaxel infused over 1 hour every 3 weeks produced an objective response rate of 49% (95% confidence interval, 40% to 58%). Two of these trials specifically included patients who had progressed while receiving either an anthracycline or an anthracenedione; the overall response rate in this subset of 83 patients was 48%. The most significant acute toxicity noted in these trials was neutropenia. Grade 4 neutropenia occurred in the majority of patients but rarely resulted in treatment delays. Hypersensitivity reactions also were common in nonpremedicated patients, but were rare after the institution of premedication with antihistamines and/or glucocorticoids. A novel toxicity observed in many patients was fluid retention syndrome, with onset at a median of four to five cycles. The fluid retention was of noncardiac or renal origin, was slowly progressive with additional cycles of therapy, was reversible after cessation of the drug, and could be largely ameliorated by oral diuretics and glucocorticoid premedication. Phase III studies to further define docetaxel's role in the treatment of breast cancer are now under way.