The aim of this study was to investigate the adjustment amount for outpatient services of clinics and to identify the factors affecting the variation of the cost between clinics in National Health Insurance in South Korea. Adjustment rate in the process of claims data review was defined as the percentage of the adjusted amount in the total claimed amount. From a total of 23,593 clinics in South Korea, 4,160 clinics accounting for 17.6% of total were selected for the study. The National Health Insurance claim data were collected during April 2007. To identify factors affecting the variation in adjustment rate between clinics, multiple regression method was used for the analysis. Older physicians were more likely to have high adjustment rate. General practitioners, orthopedic surgeons, and family physicians were more likely to have higher adjustment rate than those of internists. Physicians who have practiced between 1 and 10 years and physicians practicing in metropolitan areas had lower than their counterparts. There was a great variation in adjustment rate among physicians and the variation was affected by physicians' clinical behaviors as well as the characteristics of the clinics. Therefore, introduction of an effective management scheme for physicians' clinical behaviors is essential. Since many review adjustment occur due to the incomplete understanding about the review criteria and rules and procedure of claiming, continuing education will be a way of preventing these unnecessary review adjustments. The fact that no review adjustments were found even from the close-review implies that the current two-tiered review system (close-review and indicator-review) is an efficient way of review. Future studies are needed to classify possible cause of review adjustment such as simple errors, errors in applying the benefit scheme, and over use of services.