This paper proposes an approach to the prioritisation of system changes that takes account of the relative costs and benefits of those changes and the risks that they reduce or introduce. This is part of the SERUM methodology (Software Engineering Risk: Understanding and Management), which is being developed to help identify effective ways of using risk analysis and control in software production. SERUM introduces risk management at the initial business analysis stage of requirements investigation, and assumes an evolutionary approach to software delivery. Prioritisation is determined from five factors: benefits, costs and risk exposure in the current system, target system, and development process. The relative importance of these factors is adjustable. Results from a case study at NEC illustrate the prioritisation process and a supporting software tool is also described.