Emergency department (ED)-based observation units are becoming increasingly used for the assessment and treatment of patients who may not require inpatient management or monitoring. This is a retrospective study of 5,714 patients seen in an ED observation unit from October 16, 1996 through July 12, 2000. Of the 5,714 patients seen in the observation unit, 4,191 were discharged and 1,314 were admitted. The average length of stay was 14.92 hours. The largest group of patients seen in the observation unit was those with chest pain (almost 26%). Patients suffering abdominal pain/colic represented the second largest group (almost 16%). Pediatric patients had the shortest stay, averaging only 11.2 hours, although adult and geriatric patients averaged 15.1 hours and 15.4 hours, respectfully. The ED-based observation unit can be of great value to patient care. Although not a substitute for an inpatient unit, it is meant to be a convenient location from which to observe patients who exhibit questionable admitting criteria.