Managed care organizations rely on incentives that encourage physicians to limit medical expenditures, but little is known about how physicians respond to these incentives. We address this issue by analyzing the physician incentive contracts in use at a health maintenance organization. By combining knowledge of the incentive contracts with internal company records, we examine how medical expenditures vary with the intensity of the incentive to cut costs. Our investigation leads us to a novel explanation for high-powered group incentives: such incentives can improve efficiency in the allocation of resources when the allocation process is based on the professional judgment of multiple agents. Our empirical work indicates that medical expenditures at the HMO are 5 percent lower than they would have been in the absence of incentives.