Objectives : The purpose of this study is to analyze the proportion of admission via the emergency room(the rest is ER) in an emergency medical center and to examine the factors related to admission. Methods : This study used 2005 National Health Insurance claims data for admitted patients of 112 hospitals having emergency medical centers in Korea. The study sample had 2,335,610 patients. The data was classified into emergency admission and non-emergency admission. To investigate the factors affecting the type of admission, the following were included as independent variables: type of health assurance_(national health insurance beneficiaries or medical aid beneficiaries), demographic characteristics_ (sex, age), cause of admission_ (disease or injury), whether an operation was performed or not, DRG severity level, the number of beds, and the location of the hospital. Data were analyzed using the Chi-square test for the differences in emergency admission rates for each variables, and multiple logistic regression analysis was used for identifying the factors affecting admission type. Results : The proportion of admission via the ER accounted for 40.6% of the total admission among hospitals having emergency medical centers. The risk of admission via ER was relatively high for patients who were male, the aged, the injured, the surgical patients, the patients having more severe symptoms, and the patients admitted the hospitals located in metropolitan areas, and the patients admitted the hospitals having 300-699 beds. Medical aid patients were more likely admitted through the emergency room than health insurance patients after other variables ware adjusted. Conclusions and Discussion : We analyzed the proportion of admission via the ER for the total admission rate of hospitals having an emergency medical center in Korea. And we explored the factors related to admission via the ER. This proportion may be used as an indicator of the adequacy of medical utilization or low accessibility to hospitals of patients with low socioeconomic status.