We report a family with branchial myoclonus, spastic paraparesis, and cerebellar ataxia in which six members were affected in two generations and the inheritance appeared to be autosomal dominant. Age at onset ranged from 40 to 50 years. Rhythmic myoclonus involving the palate, pharynx, larynx, and face was followed by truncal ataxia and spastic paraparesis in most patients. CT and MRI revealed mild atrophy of the cerebral and cerebellar cortex and severe atrophy of the medulla and spinal cord. The pons appeared normal and the olives not hypertrophic. CSF studies revealed severe reduction of the serotonin metabolite 5-hydroxyindoleacetic acid. Treatment with 5-hydroxytryptophan and carbidopa at highest tolerated dose mildly improved ataxia but did not modify the myoclonus. Treatment with anticholinergics, benzodiazepines, phenytoin, valproate, carbamazepine, and baclofen was unsuccessful. The clinical symptoms were progressive, leading to death or severe disability 5 to 10 years after the onset of the disease.