A Phase I trial (NCT00109109) of oral vorinostat 200, 250 or 300 mg twice daily for 5 days/week/4-week cycle or 200, 300, or 400 mg twice daily for 14 days/3-week cycle until progressive disease or intolerable toxicity was conducted. Patients with measurable, relapsed/refractory multiple myeloma were eligible. The objectives were to determine maximum tolerated doses (MTDs) and assess activity and safety. Thirteen patients (median age, 63 years; median prior therapies, 3) were enrolled. MTDs were not determined due to early study termination by sponsor decision. One patient (250 mg twice daily 5 days/week) developed dose-limiting toxicity (DLT; grade 3 fatigue). There were no other DLTs and the maximum administered doses were 250 mg twice daily for 5 days/week/4-week cycle and 200 mg twice daily for 14 days/3-week cycle. Drug-related adverse experiences included fatigue, anorexia, dehydration, diarrhea, and nausea and were mostly grade ≤2. Of 10 evaluable patients, 1 had a minimal response and 9 had stable disease, demonstrating modest single-agent activity in relapsed/refractory multiple myeloma.