We reviewed clinical features of 48 patients without prior epilepsy who had generalized electrographic status epilepticus (ESE) identified from EEG readings. Clinical status epilepticus (SE) had occurred in 40%, but most were thought to have stopped seizing. Overall, ESE was unsuspected in 81% of patients. Many types of seizures were present before the EEG, but clinical seizures recurred in only 17 patients after diagnosis, and 18 patients (38%) never had clinical evidence of seizures. Most patients (60%) were comatose. The cause of ESE often remained uncertain though many patients had serious medical illnesses. Eighty-eight percent died, and a poor outcome was predicted by coma at the time of diagnosis (29 of 29), anoxia (15 of 15), an invariantly discharging, low-voltage background EEG (15 of 16), and refractory electrographic or clinical seizures (29 of 31). Seizure type did not predict outcome, and mortality was as high for patients without clinical evidence of seizures (89%) as for the series as a whole. ESE is often unanticipated and signifies neurologic disease with a prognosis worse than convulsive SE or the typical nonconvulsive SE; the absence of clinical seizures confers no advantage.