Hemiballism, a condition that sometimes follows an infarction or injury to the subthalamic nucleus, most often resolves spontaneously. If it does not, a ventrolateral thalamotomy may provide dramatic relief of the sometimes violent involuntary movements. The abnormal movements of ballism or hemiballism consist of involuntary hurling, irregular, frequently violent movements of the shoulder and proximal arm, which occur constantly while the patient is awake. The movements are usually the result of a lesion in the subthalamic nucleus, and consequently, the problem is ordinarily unilateral. A rare patient with hemiballism will not have a lesion in the subthalamic nucleus on autopsy, but may have involvement elsewhere in the extrapyramidal system. Since hemiballism is frequently transient, surgery should not be considered unless the problem has been stable for at least 2 or 3 months, or if the movements are so violent the patient would become exhausted before that time. Since the proximal hurling movements of hemiballism are frequently disabling, stereotactic treatment may be indicated. There is an animal model of hemiballism that occurs on production of a lesion involving 20 percent of the subthalamic nucleus in the monkey. If the lesion is enlarged to involve most of the subthalamic nucleus, the hemiballism may disappear, or it can be alleviated by interrupting the pallidofugal system. Hemiballism is of stereotactic interest from two standpoints. Not only may it be treated stereotactically, but it has been reported as an uncommon complication of the stereotactic production of thalamic lesions, presumably because of involvement of the subthalamic nucleus. The incidence after surgery ranges from 0.3 to 9.0 percent, and most occurrences resolve spontaneously in 3 weeks to 3 months. Surgical intervention involves production of a lesion in the same part of the ventrolateral nucleus or Forel's field that is the target for other movement disorders, although other targets have been used in the past, such as the globus pallidus. Results of surgery are satisfactory in 50 to 69 percent of patients. Conditions that produce neurological deficits in addition to the hemiballism may increase the surgical risk.