This study was written to discover the changes that may exist in the contents of medical services after introduction of health insurance system, and to identify the net-effect of health insurance system on medical services. Uncomplicated nornmal delivery and appendectomy patients were divided into 4 groups, the non-insured in pre-NHI periods(group A), the insured of health insurance for employees in pre-NHI periods(group B), the insured of regional health insurance for city residents in post-NHI periods(group C) and the insured of health insurance for employees in post-NHI periods(group D). The mehtod of matching was applied to control for major demographic differences among these 4 groups of each disease. In pre-NHI period, the medical services and the variation of medical services of the non-insured were compared with those of the insured. The difference between the change of medical services from group A to those of group C, and the change of medical services from group B to those group D is defined as the net-effect of health insurance. The results are as follows. First, in length of stay after delivery or operation, total length of stay, some laboratory examination, amount of several drugs used in appendectomy patients, frequency of sitz bath in delivery patients, there was net-effect of health insurance in increasing direction. Second, length of stay after delivery or operation, total length of stay, some laboratory examination, amount of several drugs used in appendectomy patients and frequency of sitz bath in delivery patients were significantly more in the insured than in the non-insured group in pre-NHI period. Third, the variation of medical services of post-NHI period was not less then those of pre-NHI period. Fourth, antenatal care on which the third party does not pay and the patient pays for all, was diffrerent by socioeconomic and educational level of patients.