While all delirious patients have clouding of consciousness (alteration of attention) and cognitive dysfunction, the level of alertness of different patients may range from stuporous to hyperalert. We, therefore, developed an analog scale to rate the alertness of delirious patients, and a separate scale to rate the severity of their clouding of consciousness. Based on these scales, patients were categorized overall as relatively “activated” (relatively alert despite clouding of consciousness), or “somnolent” (relatively stuporous along with clouding of consciousness). Cognitive function was estimated using the Mini-Mental Status Exam. Separate ratings were made of hallucinations, delusions, illusions, and agitated behavior. Activated and somnolent patients had similar ages, overall severity of delirium, and Mini-Mental Status Exam scores. Activated patients, however, were more likely to have hallucinations, delusions, and illusions than somnolent patients, and were more likely to have agitated behavior. Patients with hepatic encephalopathy were more likely to have somnolent delirium, while patients with alcohol withdrawal appeared more likely to have activated delirium. These data indicate that phenomenologic subtypes of delirium can be defined on the basis of level of alertness. These subtypes are validated in part by their differing associations with symptoms unrelated to alertness. These subtypes may have different pathophysiology, and thus, potentially different treatments.