We investigated the microtubulin inhibitor vinflunine—with trastuzumab in human epidermal growth factor receptor-2 (HER2)-positive patients—as first-line metastatic breast cancer therapy. HER2-negative patients received vinflunine on day 1; HER2-positive patients received vinflunine/trastuzumab every 21 days. Forty-eight patients in each treatment group were planned; the sponsor terminated the study early. Thirty-two evaluable patients (vinflunine, 11; vinflunine/trastuzumab, 21) were enrolled. In HER2-positive patients, vinflunine/trastuzumab produced an objective response rate (33%), clinical benefit rate (71%), and progression-free survival (6.2 months). Grade-3/4 neutropenia occurred in 14 (44%) patients; gastrointestinal toxicities were common and six patients were hospitalized for treatment-related toxicity. The vinflunine/trastuzumab combination was active and well tolerated, but our results do not suggest advantages over taxane/trastuzumab or vinorelbine/trastuzumab.