This article reports on the introduction of a cost-sharing health care plan to the United Mine Workers. The authors discuss the data base and analyze the impact of the program on the use of health care, the probability of a hospital admission, hospital expenditures and length of stay, the demand for physician services, and the probability of seeing a physician. Hospital admissions and hospital expenditures per stay decreased, as did the probability of seeing a physician. It is suggested that these behavioral adjustments to cost sharing are fairly rapid and long lasting.