A multi-institutional early phase II study of KW-2307 (vinorelbine), a new vinca alkaloid derivative, in advanced or recurrent breast cancer was conducted in 15 nationwide hospitals. KW-2307 was intravenously administered once weekly at doses of 15 to 25 mg/m2. Sixty-five among the enrolled 69 patients were eligible. Response rates were 11.8% (2/17) with 15 mg/m2, 28.0% (7/25) with 20 mg/m2 and 17.4% (4/23) with 25 mg/m2, and the overall response rate was 20.0%. Once-weekly intravenous administration of 20 mg/m2 was estimated to be the optimal dose of KW-2307 from the results. The major side effect was leucopenia, which was the dose-limiting factor in this study. Other subjective or objective side effects included anorexia, nausea-vomiting, phlebitis, fever, general fatigue and stomatitis, but none of them was serious.