These three criteria must all be present for a diagnosis of M.E./PVFS to be made. If any of these are not present the volunteer research subject should not be used for the purpose of research into M.E./PVFS and an alternative diagnosis should be keenly sought.

a) Exercise-induced fatigue precipitated by trivially small exertion -physical or mental - relative to the patient's previous exercise tolerance.

b) Impairment of short-term memory and loss of powers of concentration, usually coupled with other neurological and psychological disturbances such as emotional lability, nominal dysphasia, disturbed sleep patterns, dysequilibrium or tinnitus.

c) Fluctuation of symptoms, usually precipitated by either physical or mental exercise (see b) above.

These symptoms should have been present for at least 6 months and should be ongoing.

Although M.E./PVFS typically follows an infection, usually a viral illness (which may be subsubclinicalal) in a previously fit and active person, it has also been observed to be triggered by other factors such as immunisations, traumas and exposure to chemicals. Furthermore, in a minority of patients, M.E./PVFS has a gradual onset with: no apparent triggering factor. For these reasons proof of a preceding viral illness is not a prerequisite for diagnosis or inclusion in a study group.