On the basis of an extensive search of the literature relating to chronic fatigue syndrome (cfs) we were able to distinguish four types of explanatory models: biomedical, psychosocial, psychoanalytic and health-psychological. Because there is as yet no single definitive causal theory for cfs, doctors and patients currently use these four models to characterise the root causes of cfs. Starting form a theory of knowledge and a theory on individuals' explanations for being ill, we investigate how doctors and patients choose a particular model and we indicate to what extent the physician-patient relations can influence this choice.