Docetaxel has consistently demonstrated its high activity as an antineoplastic agent in the treatment of metastatic breast cancer. However, 90% of patients receiving the recommended dose of 100 mg/m2every 3 weeks will develop grade 3 or 4 neutropenia. Recent data suggest that the safety profile of a weekly docetaxel regimen compared favorably with the standard 3-week schedule. Thus, we initiated a phase II study to assess the efficacy and toxicity of weekly docetaxel in pretreated patients with metastatic breast cancer. Twenty patients with advanced, anthra-cycline-refractory breast cancer were included in this phase II trial. Docetaxel was administered at a starting dose of 40 mg/m2, repeated once a week for 3 consecutive weeks followed by a 1-week rest period (1 cycle). Patients were evaluated for tumor response every 8 weeks (after every other cycle). Therapy was continued for a maximum of six courses in patients showing tumor response or stable disease. Twenty patients received a total of 204 weekly infusions of docetaxel. The mean number of treatments was 10.2 (range 1–18). Eighteen patients were assessable for response. Five patients achieved a partial response and six patients showed either stable disease or a minor response. Seven patients had disease progression. The median survival was 7.8 months. Grade 3/4 leukopenia occurred in two patients. No other grade 3 or 4 hematologic toxicities were observed. The following grade 3/4 non-hematologic toxicities were seen: nausea/vomiting (one patient), infection (one patient), mucositis (two patients) and diarrhea (one patient). Three patients withdrew from the study due to dose-limiting toxicities (one due to severe neutropenia and two due to mucositis). We conclude that administration of docetaxel at a dose of 40 mg/m2was effective and well tolerated even in heavily pretreated patients with metastatic breast cancer. This regimen is associated with only mild myelosuppression.