This study outlines a model to predict hospital utilization at the small area level within a National Health Service (NHS) institutional context. The proposed approach departs from alternative analyses based on utilization flows of hospital care between a local population and a hospital. A flow demand model is outlined that relates flow demand to utilization flows; models the interaction between hospital supply and utilization of alternative hospitals; captures the process of demand for hospital care, with special attention given to the role of other health care sectors, to the organizational and institutional context of the hospital system and to geographic variations. The flow approach partly overcomes the problem of dealing with simultaneity of determination between supply and demand. A two-part econometric model suitable to estimate the flow demand model for prediction purposes is tested and applied to the Portuguese health care system. The results show the model to be robust and to provide key information for defining future hospital policies at the central level.