Primary symptoms Clinically evaluated, unexplained, persistent, or relapsing chronic fatigue that is: Of new or definite onset (has not been lifelong); 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; Is associated with post-exertional malaise (PEM)*. Additional symptoms The concurrent occurrence of three or more of the following symptoms: Substantial impairment in short-term memory or concentration; Sore throat; Tender lymph nodes; Muscle pain; Multi-joint pain without swelling or redness; Headaches of a new type, pattern, or severity; Unrefreshing sleep. These symptoms must have persisted or reoccurred during 6 or more consecutive months of illness and must not have started before the fatigue. * Modified for use in clinical diagnosis of ME/CFS, to include PEM as compulsory symptom (EUROMENE recommendation). Source: Fukuda et al. 1994 [27].