With the prospect of rapidly growing health insurance expenditures, particularly spending for ambulatory care, the introduction of a case-based payment method is discussed as an alternative to the current fee-for-service based method. A system to measure case mixes of providers is a core component of such payment systems. The objective of this study were to develop a classification system for ambulatory care, Korean Ambulatory Patient Group (KAPG) based on the U.S. APG version 2.0 and to evaluate the classification accuracy of the system. A database of 64,258,386 records was constructed from insurance claims submitted to the Health Insurance Review Agency (HIRA) during three months from August 2002. A total of 41,347,307 records with a single visit was used for the development and 7% random sample of the database was used for the evaluation. Additional groups were defined to include both physician and hospital fees in the classification, age splits were added to classify the entire population as well as the population older than 65, and the definition of medical groups used by the HIRA was adopted. The variance reduction in charges achieved by KAPGs was computed to evaluate the accuracy of classification. A total of 474 KAPGs was defined compare to 290 groups in the U.S. APG. The variance reduction for charges of all visits ranged from 20% to 37% depending on the type of provider, and ranged from 22% to 42% for non-outliers, that were better than those achieved by the system currently used by the HIRA for its internal review purpose. Although further study is required to improve the classification for complicated care in larger hospitals, the results indicated that KAPGs could be used for better management of costs for ambulatory care.