We describe a kindred with 7 confirmed and 2 probable cases of subacute dementia accompanied by myoclonus. The inheritance pattern is consistent with autosomal dominance and shows anticipation. The pathological changes involve marked gliosis with neuronal loss of the dorsomedial and midline thalamic nuclei, with lesser involvement of the anterior, lateral, and posterior thalamic nuclei. Medullary olivary hypertrophy is prominent. Spongiform change is minimal or absent. Attempted disease transmission to primates from 3 affected family members has been unsuccessful; Western immunoblot tests have likewise failed to detect the presence of specifically reactive 27-30 kD Mr proteins in brain tissue from 2 of these patients. We conclude that this family has a rare inherited neurological systems degeneration with associated thalamic dementia, the clinical course of which is very similar to Creutzfeldt-Jakob disease.