This paper focuses on lay and professional ideas about the nature of chronic fatigue syndrome (CFS), and in particular, the ways in which understandings of the disorder are developed in a clinical setting. Our data are drawn from observations of consultations between sufferers and physicians in a UK medical out-patients clinic. We treat the clinic as a political field. That is to say, as an arena in which 'problems' (about the management of illness) are constituted, and alternative approaches and solutions to such problems are pressed. We note that in the realms of symptoms, aetiology and treatment evaluation, lay people in the CFS clinic have quite distinct ideas about what their problems are and how they might be analysed and managed--ideas that are often in conflict with those of medical professionals. Thus, lay sufferers, for example, operate within a different conceptual terrain from that of many professional experts. They are more likely to refer to a disease (myalgic encephalomyelitis or ME), rather than a syndrome. They call upon different kinds of hypotheses to explain their symptoms. They hold to conflicting ideas about the order of causal sequences, and they give emphasis to different kinds of phenomena in their accounts of illness. As a consequence, clinical consultations can often take on the form of a political contest between physician and patient to define the true and real nature of the patient's disorder--a micro political struggle in which neurological symptoms can be re-framed as psychiatric symptoms, and psychiatric symptoms as neurological. In short, a contest in which the demarcation lines between mind and body are continually assessed and re-defined, and the tenets of 'biomedicine' are constantly challenged.