Cerebellar biopsy specimens were obtained at the time of cerebellar electrode installation in three epileptic patients. Cerebellar autopsy specimens also were examined from four epileptic patients and from five patients without epilepsy or neurologic disease. All specimens from seizure patients showed isomorphic gliosis of the cerebellar cortices. Significantly lower Purkinje cell densities were found in epileptic patients as compared with nonepileptic control patients. Our two epileptic patients showing marked Purkinje cell loss before cerebellar stimualtaion appeared to have better seizure control during stimulation than the patient who had only mild reduction in Purkinje cell density. These data suggest that augmentation of Purkinje cell inhibitory discharges in not the dominant mechanism for seizure suppression during cerebellar stimulation. Comparison of cerebellar specimens obtained during electrode installation with those obtained later during electrode revisions may have been prognostic significance.