This study was performed in a rural community, Kanghwa county which was introduced to a regional medical insurance pilot program in 1982. The purposes of this study were, firstly, to observe the changes in ambulatory care utilization in the three years 1982, 1983 and 1987 : secondly, to analyse factors which convert perceived medical care needs to effective medical care demand. During the three periods, a serial interview survey was performed to determine the changes in medical utilization before and after the regional medical insurance program implementation. The number of subjects was 3,356 persons in the year 1982, 3,705 in 1983 and 2,745 in 1987. The results of the study were as follows : 1. Total ambulatory care utilization rates per 100 persons during a 2-week period were 23.6 in the year 1982, 21.8 in 1983, and 29.3 in 1987; and physician visit rates were 6.1 in 1982, 11.7 in 1983, and 14.9 in 1987. Thus, compared to the total utilization rate there was a definite increase in physician visit, and during the study periods there was a decrease in drug store visits whereas an increase in hospital or clinic visits was noticed. 2. The rates of effective demand for medical care need were 70.7% in 1982, 70.5% in 1983 and 75.9% in 1987 : and the rates of patients who visited physicians were 20.2% in 1982, 42.8% in 1983 and 35.6% in 1987. Thus, physician visits increased sharply by introducing the medical insurance program, but after the latent medical care demands were fulfilled, there was a slight decrease in the physician visits. 3. The number of acute symptoms and the number of chronic symptoms were common determinants of total ambulatory care utilization and physician visits. Besides the medical care need factors, age in 1982, sex and accessibility in 1983, and accessibility in 1987 were statistically significant determinants of the total utilization ; sex and accessibility in 1983, and education in 1987 were also statistically significant determinants of the physician visit. 4. For persons with perceived acute symptoms during the 2-week periods, accessibility in total utilization and age in physician visits were common discriminating factors of ambulatory care utilization in the three years, and education and income were also statistically significant variables. For persons with perceived chronic symptoms, occupation and income were statistically significant discriminating variables commonly observed in total utilization and physician visits.