Much emphasis is now being placed on the quality of medical care, and various ways are being developed to assess the medical knowledge of general practitioners. It is increasingly recognised that the users perspective on health care is important, and that the views of health care professionals do not and cannot represent patients' views. In order to explore whether or not a large-scale survey, which asked people to rate their doctors' medical knowledge, yielded meaningful results, this paper draws on findings from a study involving in-depth interviews with 26 lay people who had already completed the General Practice Assessment Survey questionnaire. When completing the questionnaires, patients had been asked to consider the 'technical care' provided by their general practitioners and to make a judgement about their doctors' medical knowledge. When interviewed at a later date, some people explained that they defined medical knowledge as knowledge of 'disease and treatments', while others defined it as knowledge of the 'whole person', and some defined a knowledgeable doctor as one who would acknowledge uncertainty. Patients appeared to have made judgements about their general practitioners' medical knowledge based on many factors, such as their experience of illness, perceptions of professional training, contact with other health care professionals in both primary and secondary care, and exposure to the media. The paper discusses the nature of medical knowledge, and concludes that although patient surveys are useful for the evaluation of interpersonal care and access to care, asking patients about their general practitioners' medical knowledge may yield invalid results. This is partly because patients defined medical knowledge in different ways, and partly because it appears that relatively few patients had enough knowledge about their own particular illnesses, or about possible alternative treatments, to make informed judgements about their general practitioners' medical knowledge.