We evaluated unpredictable critical conditions of patients treated in the Emergency Department (ED) observation unit, who were transferred into the emergency resuscitation room from January 1 through June 30, 2001. A total of 175 patients were observed for the following critical conditions: dyspnea (51 patients; 29.14%), hypotension (28; 16.00%), chest pain (18; 10.29%), dysrhythmia (15; 8.57%), hematemesis (15; 8.57%), altered mental status (12; 6.85%), shock (10; 5.71%), coma (8; 4.57%), apnea (5; 2.86%), hematochezia (3; 1.72%), seizure (3; 1.72%), and others (7; 4.00%). The 27 patients who had cardiopulmonary resuscitation (CPR), endotracheal tube intubation, or cardioversion/defibrillation in the ED suffered an in-ED mortality of 25.9% (7) and an in-hospital mortality of 59.2% (16). The remaining 148 patients who received appropriate treatment, except for the above, had a lower in-hospital mortality (20.28%, 30 patients) (p < 0.05). We should limit the number of patients in the observation unit to avoid overloading, and classify patients according to their clinical conditions. We should determine whether or not they have definite diagnoses or are waiting for hospital admission while receiving simple treatments. The observation unit must be provided with well-trained staff and suitable physical facilities with support services, and rapid specialty consultations must be available.