Ten of 15 patients subjected to chronic cerebellar stimulation have had previously intractable seizures modified or inhibited up to periods of three years. Stimulation of anterior lobe appears to be more efficacious than stimulation of posterior lobe. Cerebellar biopsies, obtained in five patients at the time of stimulator placement, revealed in every instance a reduction in the molecular layer, decreased or absent Purkinje cells, and decreased stellate cells. One unimproved patient died during a seizure 17 months after stimulation was initiated. Histological examination of the brain did not reveal tissue damage attributable to the stimulator. There is no evidence of any adverse effect of chronic cerebellar stimulation in humans who have undergone stimulation for periods up to three years.