This study examined the changes in the service mix of Korean hospitals in 6 metropolitan cities between 2003 and 2005, and assessed whether the sample hospitals exhibit consistent trend or chance variation in multiple years. Three measures of hospital service mix, focusing on the specialization of services, were applied: information theory index, internal Herfindahl index, and number of distinct diagnosis-related groups (DRGs) treated. National Health Insurance claims were used to calculate the indexes. Specialization indexes were calculated in each year, and then examined to identify the pattern over time. Kappa analysis was applied to assess the agreements of specialization score between two years after hospitals were categorized into 4 groups with quartiles. Kappa score showed that the service mix of hospitals were changed during the study years. Specialization scores were increased given the market structure for three years. Hospitals which showed higher or lower specialization scores than the average of the scores consistently classified into the same group. Specialization indexes showed relatively consistent pattern over 3 years, and such consistencies were evident for hospitals regardless of the specialization status. Policy makers can identify the degree of specialization with the indexes, and it could provide a picture of how hospital services were mixed and changed over time.