Twenty three patients with hemiballism and two with biballism were studied. Ischaemic and haemorrhagic strokes were the cause in most patients. Other causes were encephalitis, Sydenham's chorea, systemic lupus erythematosus, basal ganglia calcifications, non-ketotic hyperglycaemia, and tuberous sclerosis. Neuroimaging studies showed a lesion of the subthalamic nucleus in only six patients. In others, different subcortical structures were involved or the results were normal. Only two patients had "pure" hemiballism. The others had other types of dyskinesias, mainly chorea, which was present in 16 patients. The prognosis was usually good.