In an open-label randomised phase III-trial patients with metastatic HER2-positive breast cancer naive to chemotherapy with normal organ function and WHO performance status < 3 are randomised to receive either docetaxel 100 mg/m2 i.v. plus trastuzumab 6 mg/kg (8 mg/kg loading dose) q 3 weeks or vinorelbine 30 or 35 mg/m2 days 1+8 plus trastuzumab 6 mg/kg (8 mg/kg loading dose) q 3 weeks. Primary endpoint is time to progression. Secondary endpoints include overall survival, time to treatment failure, response rate, duration of response and toxicity. The study hypothesis is that docetaxel is more efficient than vinorelbine but also more toxic.