The involvement of consumers in health research is now Department of Health policy within the UK. Despite the existence of policy directives, there is a dearth of knowledge on the effects of such involvement. This paper critically reviews the state of our knowledge on this issue, and maps out a research agenda with the aim of stimulating systematic, empirical inquiry into consumer involvement in health research. The paper discusses definitions of 'the consumer'; considers why consumer involvement is believed to be important to health research; traces the development of the policy; analyses the epistemological and methodological implications of the policy; discusses the various levels of consumer involvement in research; and outlines the objections to the policy that have been put forward by clinicians and researchers. Four questions were identified during the review as being in need of theoretical and empirical attention: (1) how can consumer involvement in health research be further conceptualised? (2) how and why does consumer involvement influence health research? (3) how can the influence of consumers in health research be measured and evaluated? and (4) what factors are associated with 'successful' consumer involvement in health research?