A number of focal neurologic abnormalities may accompany severe, nonketotic hyperglycemia, but extrapyramidal movement disorders have not previously been described. We evaluated the conditions of three patients with marked hyperglycemia in whom hemichorea or ballism developed that resolved completely with normalization of the blood glucose level. Potential pathogenetic mechanisms include relative dopaminergic hypersensitivity, impaired synthesis of acetylcholine or gamma-aminobutyric acid, or an undefined effect of hyperosmolarity, perhaps unmasking a previously subclinical lesion of the basal ganglia. Serum glucose level should be determined in anyone with the new onset of choreoathetosis or ballism, as hyperglycemia is a rapidly reversible cause of these conditions.