There is a growing recognition of the importance of evidence to support allocative policy decisions in health care. This study is based on interviews with politicians in four regional health authorities in Sweden. Drawing on theories of strategic use of knowledge, the article analyses how politicians perceive and make use of expert knowledge represented by the National Guidelines, embracing both a scientific and a political rationale. As health care is an organisation with a dual basis for legitimacy - at the same time a political and an action organisation - it affects knowledge use. We investigate how the context of health care priority setting influences the conditions for knowledge use among regional politicians. Our findings illustrate the dilemma of political decision-makers and how they prefer to use expert knowledge. The politicians use this policy instrument in a legitimising fashion, as it will fit into the current political debate on more equal care. As an instrument for resource allocation the politicians noted that 'facts' per se could not provide them with a sufficient basis for legitimising their governing of health care. The dualistic organisational context makes knowledge important as a political weapon in negotiations with the medical profession.