AbstractA paper in a previous issue of this journal described the development of a decision-support system for planning services within hospitals, the Hospital Planning Model. A crucial test of such a system is the actual change in the planning process its use brings about. Does it change and improve the planning process or do planners prefer to continue with historical practice? To demonstrate the potential impact of a decision-support system on planning, this paper describes a major application of the system undertaken in an inner London DHA. In this study, use of the system helped overturn management's original intention and resulted in savings of around£2 million.