There is continuing interest in action research in health care. This is despite action researchers facing major problems getting support for their projects from mainstream sources of R&D funds partly because its validity is disputed and partly because it is difficult to predict or evaluate and is therefore seen as risky. In contrast traditional health science dominates and relies on compliance with strictly defined scientific method and rules of accountability. Critics of scientific health care have highlighted many problems including a (...) perpetual quality gap between what is publicly expected and what is deliverable in the face of rising costs and the cultural variability of scientific medicine. Political demand to close the quality gap led to what can be seen as an elitist reform of policy on UK health research by concentrating more resources on better fewer centres and this may also have reduced support for action research. However, incompetent, unethical or criminal clinical practice in the UK has shifted policy towards greater patient and public involvement in health care and research. This highlights complementarity between health science and action research because action research can, as UK health policy requires, involve patients and public in priority setting, defining research outcomes, selecting research methodology, patient recruitment, and interpretation of findings and dissemination of results. However action research will remain marginalised unless either scientific research is transformed generally into a more reflective cycle or there is increased representation of action research enthusiasts within the establishment of health R&D or current peer review and public accountability arrangements are modified. None of these seem likely at this time. The case for complementarity is illustrated with two case studies. (shrink)