Purpose: Sorafenib is a kinase inhibitor targeting Raf and other kinases (ie, vascular endothelial growth factor receptor [VEGFR], platelet-derived growth factor receptor [PDGFR], Flt3, and c-KIT). This study assessed its activity and tolerability in patients with recurrent ovarian cancer (OC) or primary peritoneal carcinomatosis (PPC). Methods: This open-label, multi-institutional, phase II study used a two-stage design. Eligible patients had persistent or recurrent OC/PPC after one to two prior cytotoxic regimens, and they experienced progression within 12 months of platinum-based therapy. Treatment consisted of sorafenib 400 mg orally twice per day. Primary end points were progression-free survival (PFS) at 6 months and toxicity by National Cancer Institute criteria. Secondary end points were tumor response and duration of PFS and overall survival. Biomarker analyses included measurement of ERK and b-Raf expression in tumors and phosphorylation of ERK (pERK) in peripheral-blood lymphocytes (PBLs) before and after 1 month of treatment. Results: Seventy-three patients were enrolled, of which 71 were eligible. Fifty-nine eligible patients (83%) had measurable disease, and 12 (17%) had detectable disease. Significant grade 3 or 4 toxicities included the following: rash (n = 7), hand-foot syndrome (n = 9), metabolic (n = 10), GI (n = 3), cardiovascular (n = 2), and pulmonary (n = 2). Only patients with measurable disease were used to assess efficacy. Fourteen survived progression free for at least 6 months (24%; 90% CI, 15% to 35%). Two patients had partial responses (3.4%; 90% CI, 1% to 10%); 20 had stable disease; 30 had progressive disease; and seven could not have their tumor assessed. ERK and b-Raf were expressed in all tumors. Exploratory analyses indicated that pERK in post-treatment PBL specimens was associated with PFS. Conclusion: Sorafenib has modest antitumor activity in patients with recurrent OC, but the activity was at the expense of substantial toxicity.