OBJECTIVE The National Cancer Institute of Canada-Clinical Trials Group (NCIC-CTG) conducted a phase II study to assess the efficacy and toxicity of docetaxel as first-line chemotherapy in metastatic breast cancer (MBC). METHODS Fifty-one patients with measurable MBC were studied. Three patients were ineligible and were excluded from analysis. The planned dose of docetaxel was 100 mg/m2 intravenously (i.v.) every 3 weeks. Prior adjuvant chemotherapy was allowed if at least 12 months had elapsed from completion of treatment to recurrence. RESULTS The most severe toxicity was granulocytopenia. Ten patients (20.8%) were hospitalized for febrile neutropenia. The protocol was amended to a starting dose of 75 mg/m2 for the last 16 patients. Sixty percent of patients experienced hypersensitivity reactions (HSRs). After two protocol amendments, the use of a premedication regimen of oral dexamethasone and i.v. H1 and H2 blockers prevented significant HSRs. Edema developed in 62% of patients and was cumulative, was present in 50% who received greater than 400 mg/m2, and was not improved by premedication regimens. Following an independent radiology review, 22 partial remissions and four complete responses in 47 assessable patients were confirmed (response rate, 55%; 95% confidence interval [CI], 40% to 69%). The response rate for 15 assessable patients registered at 75 mg/m2 was 40% (95% CI, 16% to 67%); for 32 assessable patients registered at 100 mg/m2, the response rate was 63%, (95% CI, 43% to 78%). CONCLUSIONS Docetaxel is an active agent in MBC. Its activity as a single agent is comparable to many combination chemotherapy regimens and is not affected by prior adjuvant chemotherapy. Studies are ongoing to improve its therapeutic index and to incorporate docetaxel in combination chemotherapy regimens.