Romidepsin is a potent histone deacetylase inhibitor; preclinical studies showed potential synergy with the nucleoside analog gemcitabine. This phase 1 trial was conducted to determine the maximum tolerated dose for two schedules of romidepsin plus gemcitabine in patients with advanced solid tumors in which gemcitabine had previously demonstrated clinical activity. The recommended phase 2 dose was 12 mg/m(2) romidepsin plus 800 mg/m(2) gemcitabine on days 1 and 15 every 28 days. Results suggest additive hematologic toxicities of romidepsin plus gemcitabine, but the level of antitumor activity observed warrants more formal trials of this combination to further assess safety and efficacy.