The study evaluated the impact of the October, 1984 introduction of a 10% cost sharing on the attitude of insured persons in the Employees' Health Insurance Scheme. The subjects were 1701 Health Insurance Societies, all of which had joined the Health Insurance Society Association from 1983 to 1985. Physician utilization rates in 1983 and 1984 were analyzed to disclose the effects of the 10% cost sharing introduction. Moreover the author examined which factors of Health Insurance Society were related to the changes of the rates by multiple regression analysis. The results are as follows. 1. All medical service rates of admissions, outpatient visits, and dental visits were reduced significantly after the introduction. 2. The impacts were higher in the Societies in which rates before the introduction were higher for all medical services. 3. Multiple regression analysis revealed differences in indicators related to the characteristics of the society among all medical service rates.