Based on preclinical data demonstrating cytotoxic synergy between sorafenib and entinostat, a phase I study of this combination was conducted in patients with advanced solid tumors. Enrollment followed the traditional "3 + 3" dose escalation scheme. Entinostat was given orally once every 2 weeks, starting at a dose of 4 mg and escalating to 6 and 10 mg every 2 weeks. Sorafenib was administered as a continuous oral dose, escalating from 200 to 400 mg twice daily. A treatment cycle was 28 days. A total of 31 patients with advanced solid tumors were enrolled on the study. The three dose-limiting toxicities (DLTs) observed were grade 3 hand-foot syndrome, nausea/vomiting, and fatigue. MTD was not reached. The recommended phase II dose was defined as the full dose of the respective drugs administered individually. The most common grade 3-4 toxicities were muscle weakness (13 %), skin rash (10 %), fatigue (6 %), diarrhea (6 %), and hand-foot syndrome (3 %). One NSCLC patient achieved a partial response. Two patients (adenocarcinoma of GE junction and Hurthle cell carcinoma of the thyroid) were on the study for more than 9 months with stable disease. The combination of entinostat and sorafenib was well tolerated. Entinostat 10 mg orally once every 2 weeks in combination with sorafenib 400 mg orally twice daily, representing full single agent doses of each drug was identified as the recommended phase 2 dose (RP2D). These data support future clinical development of the combination of entinostat and sorafenib.