These guidelines lack sensitivity and specificity and are not recommended for diagnosis.

1. Fatigue

Clinically evaluated, unexplained, persistent or relapsing fatigue persistent for six months or more, that:

• is of new or definite onset;
• is not the result of ongoing exertion;
• is not substantially alleviated by rest;
• results in substantial reduction in previous levels of occupational, educational, social or personal activities;

Four or more of the following symptoms that are concurrent, persistent for six months or more and which did not predate the fatigue:

• impaired short term memory or concentration;
• sore throat;
• tender cervical or axilliary lymph nodes;
• muscle pain;
• multi-joint pain without arthritis;
• headaches of a new type, pattern, or severity;
• unrefreshing sleep;
• post-exertional malaise lasting more than 24 hours.

The Fukuda definition can be read in full in the Annals of Internal Medicine.

Comparing the Canadian and Fukuda definitions

In contrast with the Canadian Clinical Case Definition of ME/CFS, the Fukuda definition of CFS was designed primarily for research purposes.

The Fukuda definition of CFS is less specific than the Canadian Clinical Case Definition of ME/CFS (2003). The Fukuda definition focuses primarily on fatigue and downplays the importance of other core symptoms of ME/CFS.

In the Fukuda definition fatigue is the only compulsory symptom, with four other symptoms being required out of a list of eight: impaired short term memory or concentration; sore throat; tender cervical or axilliary lymph nodes; muscle pain; multi-joint pain without arthritis; headaches of a new type, pattern, or severity; unrefreshing sleep; post-exertional malaise lasting more than 24 hours.

Using the Fukuda definition it is possible for someone to be diagnosed with "CFS" without experiencing, for example, post-exertional malaise, unrefreshing sleep or cognitive problems. The result is that the Fukuda definition tends to select people with a wide range of "fatiguing illnesses", including depressive disorders and not only people with ME/CFS.

The Canadian Clinical Case Definition better reflects the clinical aspects of ME/CFS and enables a medical practitioner to more easily distinguish ME/CFS, with its pathological fatigue, from ordinary fatigue and other fatiguing illnesses.

The Fukuda (1994) definition of Chronic Fatigue Syndrome is used in the Royal Australian College of Physicians (RACP) 'Chronic Fatigue Syndrome Clinical Practice Guidelines' (2002). ME/CFS Australia promotes the use of the Canadian Clinical Case Definition (2005).