Status epilepticus (SE) is not rare in critically ill intensive care unit (ICU) patients, but its diagnosis is often delayed or missed, in part because it is mistaken for other causes of altered mental status. Even once diagnosed, SE in the ICU can be refractory to treatment. We sought to determine the causes, clinical features, and difficulties in diagnosis of SE in the ICU, and the effects of antiepileptic drugs (AEDs) on its course. We reviewed the course of ICU patients with both clinical and EEG evidence of SE, attempting to determine which patients are at risk for unsuspected SE, what was the typical delay in diagnosis, and whether AED treatment made a difference in their clinical courses. By clinical and EEG evidence, 91 ICU patients with SE were identified, all with abnormal mental status: 74 were comatose. Vascular disease (in 24) and anoxia (22) were the most common causes; most had multiple medical problems. Although 76 patients had clinically evident seizures earlier (and 56, clinical SE) only 20 were thought to be in SE at the time of the diagnostic EEG. There was a median delay of 48 hours from clinical deterioration until diagnosis in patients with earlier clinical seizures and 72 hours without seizures. Among the 68 nonanoxic patients treated with AEDs, 38 (56%) seemed to improve in alertness, including 25 who were comatose. Although patients who were stuporous or confused (vs. comatose) improved more often on AEDs, they were less often realized to be in SE before the EEG. Patients with earlier seizures were also more likely to improve, but no more likely to be diagnosed before the EEG. Patients who responded to AEDs were more likely to survive. ICU patients with altered mental status and EEG evidence of SE often have severe medical and surgical illnesses, refractory SE, and a high mortality. The delay to diagnosis is substantial, but a significant subset of patients improves on AEDs once SE is discovered. This diagnosis should be sought more often in ICU patients with abnormal mental status, especially after clinical seizures or SE without full recovery.