Twenty five patients with metastatic breast cancer will be randomized to receive weekly trastuzumab plus either G-CSF or placebo by subcutaneous (SQ) injection daily for five days weekly for two weeks. Subsequently, all patients will receive an additional 12 weeks of weekly trastuzumab, G-CSF by SQ injection daily for five days weekly for 12 weeks, and vinorelbine once weekly at a dose of 25 mg/m2 weeks 3, 4, 6, 7, 9, 10, 12, 13. Baseline evaluation will include a history and physical exam, comprehensive metabolic panel (CMP), complete blood count (CBC), serum pregnancy test, computerized tomography (CT) scan for disease measurements, and a Multiple Uptake Gated Acquisition (MUGA) scan. The CT scan and MUGA will be repeated upon completion of the study treatment. Blood will be drawn pre-trastuzumab, 2 hours post-trastuzumab, and 48 hours post-trastuzumab on weeks 1, 2, 3, 4, and 12 to measure whole blood ADCC activity. Two additional assays for whole blood ADCC activity will be drawn at baseline pre-treatment, and following completion of protocol treatment. These assays will measure chromium release from a Her-2 positive target cell exposed to the patient's effector cells. Measurement of soluble Her-2 in patient serum will also be measured at each ADCC time point.