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The effect of cost sharing on health services utilization is analyzed from a new perspective, that is, its effects on physician response to cost sharing. A primary data set was constructed using medical records and billing files from a large multispecialty group practice during the three-year period surrounding the introduction of cost sharing to the United Mine Workers Health and Retirement Fund. This same group practice also served an equally large number of patients covered by United Steelworkers' health benefit plans, for which similar utilization data were available. The questions addressed in this interinsurer study are: (1) to what extent does a physician's treatment of medically similar cases vary, following a drop in patient visits as a result of cost sharing? and (2) what is the impact, if any, on costs of care for other patients in the practice (e.g., "spillover effects" such as cost shifting)? Answers to these kinds of questions are necessary to predict the effects of cost sharing on overall health care costs. A fixed-effects model of physician service use was applied to data on episodes of treatment for all patients in a private group practice. This shows that the introduction of cost sharing to some patients in a practice does, in fact, increase the treatment costs to other patients in the same practice who remain under stable insurance plans. The analysis demonstrates that when the economic effects of cost sharing on physician service use are analyzed for all patients within a physician practice, the findings are remarkably different from those of an analysis limited to those patients directly affected by cost sharing.