The amount of resources used in the care of chronically ill Medicare fee-for-service (FFS) patients varies widely across hospitals. We studied variations across California hospitals in hospital resource use for chronically ill patients covered by Medicare health maintenance organizations (HMOs) and private insurers and found substantial variation in all of the coverage groups studied. Resource-use measures based on Medicare FFS data often reflect patterns evident for other payers. Previous estimates of savings if the most resource-intensive hospitals more closely resembled less resource-intensive hospitals, based on just Medicare FFS spending, could underestimate possible savings when other payers are taken into account.