Background: Priority setting is necessary in current healthcare services. Discussion of fair process has highlighted the value of developing reasons for allocation decisions on the basis of experience gained from real cases. Aim: To identify the reasons that those with experience of real decision-making concerning resource allocation think relevant in deciding on the priority of a new but expensive drug treatment. Methods: Semistructured interviews with members of committees with responsibility for making resource allocation decisions at a local level in the British National Health Service, analysed using modified grounded theory. Results: 22 interviews were carried out. 14 reasons were identified. Four reasons were almost universally considered most important: cost effectiveness; clinical effectiveness; equality and gross cost. No one reason was considered dominant. Some considerations, such as political directives and fear of litigation, were thought by many participants to distort decision-making. There was a substantial lack of agreement over the relevance of some reasons, such as the absence of alternative treatment for the condition. Conclusions: There is a clear consensus on the importance and role of a limited number of reasons in allocation decisions among participants. A focus on the process of decision-making, however, does not obviate the need for those involved in the process to engage with problematical ethical issues, nor for the importance of further ethical analysis.Over the past few decades, financial pressures have led governments and healthcare organisations to adopt systematic approaches to priority setting. Priority choices involve ethical values and a persisting challenge for policy-makers is to specify an appropriate package of health services that is ethically defensible.