OBJECTIVES: This study was designed to investigate the impact of medicaid expansion on preventable hospitalization(PH) trend. METHODS: Data from the Korea National Health Insurance(NHI) claims and medicaid claims database, covering 48 million Korean people, was used.  The new medicaid people were selected who were enrolled first time during the medicaid expansion period, the year 1998-1999. The NHI enrollees were randomly selected for control group. All were continuously enrolled from 1996 to 2001 and aged more than 17 years.  Multinomial logit regression models of comparing PH versus marker admission  were estimated. Model variables included insurer's type, age, gender, charlson comorbidity index, year. Odd ratios(OR) and 95% confidence intervals(CI) were used to investigate the impact of medicaid expansion on PH trend.   The PH was defined as a class of admission believed to be preventable in most cases by standard ambulatory health care before admission. The marker admissions were determined such as appendicitis and gastrointestinal obstruction, remaining relatively constant across populations.  RESULTS: Among newly enrolled 140,366 medicaid population, 62.1% were female, mean age was 52 years old. These percentage of female and mean age among 280,301 NHI population were 53.1% and 39 years old, respectively. The biennial ratio of PH relative to marker admission in medicaid population increased after eligibility expansion, from an average of 6.05 to 9.04(OR=1.50). This rise was small compared with the NHI populations, who experienced from  1.96 to 3.22(OR=1.64). After adjusting for confounding variables, the OR for after medicaid expansion among medicaid population and NHI population were 1.48(95% CI=1.29-1.69) and 1.71(95% CI=1.53-1.91), respectively. CONCLUSIONS: Current findings suggest that the medicaid PH trend was less likely to rise than NHI PH trend after implementing medicaid expansion.