The purpose of this study was to evaluate the maximum tolerated dose and the toxicity profile of vinorelbine administered by continuous infusion for 96 hours to patients who had received prior chemotherapy for metastatic breast cancer. Forty-three patients with metastatic breast cancer were treated with vinorelbine 8 mg intravenously for 10 minutes (day 1) followed by continuous infusion of vinorelbine for 96 hours. Treatments were repeated every 3 weeks. Eighty-eight percent of the patients had had two or more prior chemotherapeutic regimens: 91% had prior doxorubicin therapy and 77% had prior paclitaxel therapy. All 43 patients were evaluable for toxicity. The median age was 49 years. All patients had a performance status less than or equal to 2 and a life expectancy more than 12 weeks. Eight dose levels were evaluated, and a total of 182 cycles were given. National Cancer Institute grade III or IV granulocytopenia was observed in 64 (35%) cycles, neutropenic fever in 27 (15%) cycles, fatigue (National Cancer Institute grade III or IV) in 18 (10%) cycles, and hand-foot syndrome in 8 (4%) cycles. In 17 (9%) cycles, patients were hospitalized. The maximum tolerated dose of this regimen was determined to be vinorelbine 8 mg intravenously for 10 minutes (day 1) followed by continuous vinorelbine infusion 11 mg/m2 for 96 hours. The dose-limiting toxicity was neutropenic fever and stomatitis.