Transient global amnesia is an unusual form of the amnestic syndrome in that it is completely reversible in most patients. The etiology of transient global amnesia is poorly understood, although involvement of the medial temporal lobe is most likely. The literature suggests that once an underlying structural lesion has been ruled out, cases can be divided into three groups: patients with a past history of migraine headaches, those who have no obvious underlying precipitating factors, and those with associated neurologic deficits during a spell and risk factors for cerebrovascular disease. Patients in the first group tend to have attacks at an earlier age and to have an increased recurrence rate but, like those in the second group, have a good prognosis. Patients in the third group often have onset later in life, and as many as 10% may have a stroke or dementia. Whether or not this is a higher incidence than should be expected in persons of this age group has not been determined. Although sophisticated testing may show persistence of the memory defect, most patients eventually recover completely.