Patients frequently present to the emergency room (ER) with mental status changes without obvious cause. The EEG is underused in this population. The authors investigated whether an abbreviated EEG (AbEEG) can be incorporated in the early evaluation of these patients to provide useful information. A 5-minute AbEEG was performed using a preformed electrode placement system on 25 patients who presented to the ER with mental status changes of unknown cause. AbEEG findings were categorized as normal, showing diffuse abnormalities, focal abnormalities, electrographic seizures, or uninterpretable. Using retrospective chart review, the authors determined if the cause of mental status change was a diffuse encephalopathy or a nonneurologic event (DENNE), a focal brain abnormality, nonconvulsive status epilepticus (NCSE), psychogenic, or unknown, and if particular AbEEG findings were associated with specific causes of altered sensorium. The AbEEG identified NCSE in two patients who presented with new-onset seizures. The presence of diffuse slowing on the AbEEG was highly suggestive of mental status changes due to DENNE. AbEEGs can be successfully incorporated in the early evaluation of patients who present to the ER with mental status changes of unknown cause and provide useful information in this setting.