In the present study I identify an inherent characteristic of health care markets that may lead to excessive investment by hospitals even when compensated according to a prospective reimbursement rule. It is demonstrated that the stochastic nature of the demand for medical services combined with the lumpiness of investment decisions may give rise to excessive investment when multiple hospitals select independently their levels of capacities. The source for the excessive incentives to invest is the difficulty of one hospital to internalize properly the externality generated by its investment decisions. Such an externality arises because when one hospital expands its capacity, it is more likely to be able to serve not only patients residing in its region but patients residing in neighboring regions as well.