BACKGROUND: The purpose of ambulatory facilities is to relieve the pressure on hospital beds and to make the process of surgery more convenient for the patient. One way of assessing the success of a ambulatory unit is to look at the number of patients requiring admission to hospital. Thus the author investigated the rate, distribution, and cause of admission cases to get help to reduce the admission rates. METHODS: Total ambulatory surgeries were 15,512 cases for the period from March 1995 to January 1998. The medical records were reviewed and the patients admitted to hospital instead of being discharged home were analyzed retrospectively. RESULTS: The overall mean rates of admissions for the period was 0.76 percent. The principal reasons for admissions were requestion by surgeon (0.53%), wish of patients (0.12%), surgery related admissions (0.08%) and anesthesia related admissions (0.03%), respectively. CONCLUSIONS: With proper assessment and selection of patients and surgical procedures, the admission rates of ambulatory surgery will be reduced acceptably low. Thus the efficiency of ambulatory unit will be increased. But it will reduce the use rate of ambulatory unit. For increasing the use rate of ambulatory unit, our hospital is broadening the patients' selection criteria.