• A 10-year-old boy with ataxia-telangiectasia had severe progressive dystonic posturing that masked the ataxia until treatment relieved the dystonia. A younger sister had more classical neurologic manifestations of the disease. However, both children had telangiectasia, immunologicabnormalities, and other features of ataxia-telangiectasia. The pathologic changes that have been found in the basal ganglia at autopsy and the occurrence of choreoathetosis, oculomotor disturbances, and now dystonia indicate that the function of the basal ganglia in patients with ataxia-telangiectasia is abnormal. Children who have basal ganglial abnormalities should be studied for ataxia-telangiectasia.