Abstract Remacemide hydrochloride is a low-affinity, non-competitive N -methyl- D -aspartic acid (NMDA) receptor channel blocker, under investigation in epilepsy. This double-blind, placebo-controlled, multicentre study assessed the safety and efficacy of remacemide hydrochloride or placebo, as adjunctive therapy, in 252 adult patients with refractory epilepsy who were already taking up to three antiepileptic drugs (including an enzyme-inducer). Patients were randomized to one of three doses of remacemide hydrochloride (300, 600 or 1200 mg /day) or placebo Q.I.D., for up to 15 weeks. An increasing percentage of responders (defined as a reduction in seizure frequency from baseline of ≥50%) was seen with increasing remacemide hydrochloride dose. At 1200 mg /day, 23% of patients were responders compared with 7% on placebo. This difference was significant ( P= 0.016), as was the overall difference between treatments ( P= 0.038). Adverse events: dizziness, abnormal gait, gastrointestinal disturbance, somnolence, diplopia and fatigue were mild or moderate in severity. Carbamazepine and phenytoin plasma concentrations were well controlled and maintained within target ranges, with no evidence of improved seizure control due to increases in the concentrations of these drugs. A dose-dependent, significant, increase in responders following adjunctive remacemide hydrochloride compared with placebo was observed. Remacemide hydrochloride was well tolerated.