We treated 14 patients with medically refractory partial seizures by stimulation of the vagus nerve in two single-blind pilot studies. Patients received stimulation through an implantable, programmable NeuroCybernetic Prosthesis, consisting of a pulse generator and a lead-electrode assembly. The mean reduction in seizure frequency after 14 to 35 months of vagal stimulation was 46.6%. Of the 14 patients, five (35.7%) had a 50% or greater reduction in seizure frequency. Two patients, one of whom had had 10 to 100 seizures per day before stimulation, have been seizure-free for over 1 year. Adverse events were primarily limited to initial hoarseness and a tingling sensation at the electrode site in the neck when the device was activated. Most patients tolerated the device and stimulation well. There were no permanent adverse events. Some cases of medically refractory partial seizures are improved by vagal stimulation.