We conducted a systematic review of all studies of status epilepticus (SE) with more than 30 patients published between January 1, 1990, and December 31, 2008, to determine the frequencies of the common underlying causes and the extent to which the underlying causes affect the prognosis of an episode of SE. The frequencies of underlying causes vary among studies and show marked geographic differences, but in most studies, the most common underlying causes were cerebrovascular disease and low antiepileptic drug levels. A relatively good prognosis of SE is found when the underlying cause is associated with low antiepileptic drug levels or alcohol abuse, and a relatively poor outcome occurs when the underlying cause is cerebrovascular disease, particularly in the case of SE due to acute cerebral anoxia, but in most conditions, the reported prognosis is variable. Also, when SE occurs in the context of an acute cerebral insult, such as cerebral infection or cerebrovascular disease, the prognosis of the acute cerebral event is worsened.