Hemiballism is a rare movement disorder that presents with unilateral flinging movements of the limbs. In traditional teaching, it has been characterised as almost pathognomonic of a lesion in the subthalamic nucleus (STN). The prognosis was described as grave, with severe disability and death in many cases. However, review of more recent reports shows that the STN is directly involved in only a minority of cases. The prognosis is benign in most cases, with almost all patients responding well to treatment and many having spontaneous remission, although long-term prognosis of cerebrovascular disease may not be so good. There have also been recent insights into the pathophysiology of hemiballism, which have emphasised the importance of altered firing patterns in basal-ganglia structures. Recent studies have pointed to previously unrecognised causes, particularly non-ketotic hyperosmolar hyperglycaemia and complications of HIV infection, that may account for a substantial proportion of cases of hemiballism.