Many policy makers believe that when Medicare constrains its payment rates for hospital inpatient care, private insurers end up paying higher rates as a result. I tested this “cost-shifting” theory using a unique new data set that combines MarketScan private claims data with Medicare hospital cost reports. Contrary to the theory, I found that hospital markets with relatively slow growth in Medicare inpatient hospital payment rates also had relatively slow growth in private hospital payment rates during 1995–2009. Using regression analyses, I found that a 10 percent reduction in Medicare payment rates led to an estimated reduction in private payment rates of 3 percent or 8 percent, depending on the statistical model used. These payment rate spillovers may reflect an effort by hospitals to rein in their operating costs in the face of lower Medicare payment rates. Alternatively, hospitals facing cuts in Medicare payment rates may also cut the payment rates they seek from private payers to attract more privately insured patients. My findings indicate that repealing cuts in Medicare payment rates would not slow the growth in spending on hospital care by private insurers and would in fact be likely to accelerate the growth in private insurers’ costs and premiums.