Objective¡G The purpose of this research is (1) to find out the overall trends of the common avoidable hospitalization diseases in Taiwan since the implementation of the National Health Insurance (NHI), (2) to compare the different distributions of the common avoidable hospitalization diseases across age groups, genders, regions, and the hospital¡¦s characters, (3) to make the related useful information and suggestions of improving avoidable hospitalizations to the policy makers, hospitals, and the general public. Method¡G Standardized rates of avoidable hospitalization from 1997 to 2004 were derived from the National Health Insurance database and the Taiwan census data. In this research, we focus on the four main diseases¡GAsthma, Pneumonia, Diabetes, Hypertension as the avoidable hospitalization diseases. Logistic regression and chi-squire tests are used to detect and describes trends in data. Results¡G During the 8-year research period, AHCs (Avoidable Hospitalization Conditions)of Asthma and Hypertension decline overall, conversely, AHCs of Pneumonia and Diabetes show increasing trends overall¡Fin the hospital characters aspects, AHCs are mostly occurred in the local hospitals¡Ffrom the viewpoint of the patients¡¦ characters, AHCs of Asthma and Pneumonia are always occurred in children male, on the other hand, AHCs of Diabetes and Hypertension are always occurred in female and above 65 years old¡Fin the regional aspect, the AHCs of Asthma and Pneumonia are comparatively higher in east Taiwan. Conclusion¡G According to the results of this research, we can find that the avoidable hospitalizations in Taiwan still need to be improved. We suggest the policy makers and the hospitals may use the ¡uavoidable hospitalization¡vas an indicator to monitor the quality of care and the outpatient care quality in Taiwan¡¦s healthcare system. Thus can not only reduce the unnecessary expenditure, but also can offer the general public better quality of healthcare and improve their quality of life.