Chronic fatigue syndrome (CFS) is characterized by severe and prolonged fatigue, affecting both physical and mental functioning, exacerbated by relatively minor exertion (Fukuda et al. 1994). A variety of other symptoms such as impaired concentration and memory, disturbed sleep, depressed mood and anxiety are also often present. Alongside this emerging consensus describing the clinical features of chronic fatigue, controversy has raged as to its aetiology, particularly the relative importance of viruses and other infectious agents, the contribution of neuromuscular abnormalities and whether the association with psychiatric disorders is primary or secondary.