In this paper, the method of rationally determining the system structure and operational strategies is proposed for the energy supply system for a hospital based on the optimization approach. First, energy demands are estimated hourly, daily, and seasonally through one year for the hospital with total floor area of 25,000 m(2). Second, miscellaneous kinds of equipment are listed up such as refrigerators, heat pumps, gas engine cogeneration unit, steam boiler, thermal storage tank, and other similar things and 25 structures of alternative energy supply systems are composed by combining these kinds of equipment. By the mathematical optimization method, the following results have been obtained; that is, the optimal system is the hybrid-system that consists of electricity-driven and natural gas-fired equipment together with cogeneration equipment, which is difficult to get by intuition and experience of the designer. Sensitivity analysis is also carried out by changing parametric values related to initial capital cost and upgrading performance of each equipment in addition to energy charges, and as the obtained result the optimal energy supply system for the hospital is proposed from comprehensive viewpoints. (C) 2007 Elsevier Ltd. All rights reserved.