We measured the activity of choline acetyltransferase, the cholinergic marker enzyme, in the brains of 17 patients from five established pedigrees with dominantly inherited olivopontocerebellar atrophy (OPCA). OPCA is a group of cerebellar ataxia disorders in which serious intellectual impairment is not typically considered to be an accompanying feature. Patients from all five pedigrees demonstrated markedly reduced choline acetyltransferase activity in the cerebral cortex, with less severe changes in the hippocampus. Although the magnitude of the cortical choline acetyltransferase deficit is comparable to that seen in the brains of patients with Alzheimer's disease, none of our OPCA patients appeared, on last examination, to have severe global dementia of the Alzheimer type. Determination of the clinical significance of our biochemical data must await the results of studies in which the cognitive status of OPCA individuals has been accurately assessed.