Rapid differentiation of organic brain syndrome (OBS) from functional psychiatric illness can be difficult when patients come to an emergency department with predominantly psychiatric symptoms. Using four screening criteria-disorientation, abnormal vital signs, clouded consciousness, and patients older than 40 years with no previous psychiatric history-we detected 38 cases of OBS from 1,140 medically cleared patients. Twelve of these patients were subsequently admitted to a medical unit. Despite the presence of striking behavioral aberration, an increased awareness of the clinical manifestations of OBS will enhance the physician's ability to discriminate OBS from other psychiatric illness.