This trial was conducted to assess the efficacy and safety of sorafenib in patients with metastatic breast cancer. In this multinational, open-label phase II study, patients with metastatic breast cancer that had progressed after at least one prior chemotherapy regimen were continuously treated with oral sorafenib, 400 mg twice daily. The primary endpoint was overall best response; a secondary endpoint was percentage of patients with stable disease for greater than or equal to 16 weeks. Biomarker analysis were also performed. Of the 56 patients enrolled into the study, 54 were treated with at least one dose of sorafenib. Partial response was observed in one patient (2%) and stable disease in 20 patients (37%); no complete responses were observed. Disease stabilization for greater than or equal to 16 weeks was seen in 12 patients (22%); stabilization for greater than or equal to 6 months in seven patients (13%). The most common drug-related grade 3 adverse events were rash/desquamation (6%), hand-foot skin reaction (4%), and fatigue (4%). Baseline vascular endothelial growth factor levels, levels of soluble epidermal growth factor receptor during treatment and both baseline and changes in soluble human epidermal growth factor receptor 2 levels correlated significantly with clinical outcomes. Although the primary endpoint of overall response rate showed minimal improvement on sorafenib 400 mg twice-daily treatment, the rate of disease stabilization was encouraging in patients treated with one or more lines of chemotherapy. The treatment had a clinically manageable toxicity profile. Further investigation of single-agent sorafenib in this patient population is not recommended; however, studies investigating combinations of sorafenib with chemotherapeutic agents are warranted and ongoing.