MS-275 is a benzamide derivative with potent histone deacetylase (HDAC) inhibitory and antitumor activity in preclinical models. We conducted a phase 1 trial of orally administered MS-275 in 38 adults with advanced acute leukemias. Cohorts of patients were treated with MS-275 initially once weekly x 2, repeated every 4 weeks from 4 to 8 mg/m2, and after 13 patients were treated, once weekly x 4, repeated every 6 weeks from 8 to 10 mg/m2. The maximum-tolerated dose was 8 mg/m2 weekly for 4 weeks every 6 weeks. Dose-limiting toxicities (DLTs) included infections and neurologic toxicity manifesting as unsteady gait and somnolence. Other frequent non-DLTs were fatigue, anorexia, nausea, vomiting, hypoalbuminemia, and hypocalcemia. Treatment with MS-275 induced increase in protein and histone H3/H4 acetylation, p21 expression, and caspase-3 activation in bone marrow mononuclear cells. No responses by classical criteria were seen. Our results show that MS-275 effectively inhibits HDAC in vivo in patients with advanced myeloid leukemias and should be further tested, preferably in patients with less-advanced disease.