The monthly ambulatory treatment days in newly detected hypertension group and known hypertension group were analyzed. The population was identified through the records of screening examination given by Korea Medical Insurance Corporation during the period from April to July, 1986. From the records of screening examination, 11,614 hypertensive patients were identified. By random sampling,959 patients were selected : among them, 544 fell under the category of known hypertension group and the other 415 fell under the newly detected hypertension group. The monthly ambulatory treatment days of these patients during the period from the April, 1985 to September, 1987 were analyzed in order to compare the exents of medical care utilization as well as to define and analyze the determinants responsible for the ambulatory treatment days between the two groups. The following results were obtained. 1) In the known hypertension group, no statistically significant changes in the ambulatory treatment days was observed after, in comparision to before, the screening examination. However, in the newly detected hypertension group the medical care utilization increased after the screening examination because of hypertension. 2) The ambulatory treatment days for hypertension of the known hypertension was statistically significant and higher than that of the newly detected hypertension group after screening examination. 3) There was no statistically significant change in the ambulatory treatment days in association with diseases other than hypertension in either group before and after the screening examination. 4) There was no statistically significant variable responsible for ambulatory treatment days in the known hypertension group. However, the income was a statistically significant variable in the newly detected hypertension group. 5) After the screening examination, the variables determining the ambulatory treatment days were the age of the patient and the diastolic blood pressure in the known hypertension group. These variables responsible for 2.02% of the total ambulatory treatment days. In the newly detected hypertension group, the income was a statistically significant variable which was responsible for 2.10% of total ambulatory treatment days. The above results satisfied the hypothesis that there would be no significant changes in the ambulatory treatment days before and after the screening examination in the known hypertension group. Also the hypothesis that there would be no significant change in the exents of medical care utilization for the diseases other than hypertension before and after the screening examination in either group was satisfied Also the medical care utilization was significantly higher in the known hypertension group than the newly detected hypertension group after the screening examination. This finding satisfied the hypothesis. This study was limited by the lack of considering fully the variables reponsible for the clinical symptoms of hypertension as well as for the individual characteristics. Thus, the result of this study are not fully adequate to define the determinants responsible for the exents of medical care utilization. In the future studies on medical rare utilization, additional variables should be considered.