Chronic fatigue syndrome or myalgic encephalomyelitis (CFS/ME) is a common (0.19–2%)1,–,4 and disabling condition in children and young people, with over 50% of affected children and young people bed bound at some stage and a mean time off school of 1 academic year.5 The importance of CFS/ME has been highlighted by both the Chief Medical Officer's report6 and the National Institute for Health and Clinical Excellence (NICE) guidelines.7

A variety of case definitions for CFS/ME exist (box 1), and studies, including those reviewed here, often use different definitions, particularly around the duration of symptoms. It is not clear whether the different case definitions have different implications for aetiology or prognosis. Paediatric CFS/ME is defined in the Royal College of Paediatrics and Child Health guidelines as “generalised fatigue persisting after routine tests and investigations have failed to identify an obvious underlying cause”.8 The NICE recommended that the fatigue should have lasted 3 months before a diagnosis is made and highlighted the fact that the fatigue must be disabling and was likely to occur in the presence of one other symptom7 (box 1). There is a philosophical discussion to be had on how diagnoses are made, particularly with diagnoses where no underlying pathogen has been identified, but it can be argued that “delineation of a syndrome is the first step towards elucidation of the chain of causation and redefinition of the whole group or subgroups within it”.9