The criteria for CFS require 5 symptoms (fatigue plus 4 more) not 4 symptoms as were assessed Tom Kindlon, Irish ME/CFS Association - for Information, Support & Research 15 August 2011 Bhui and colleagues  state: "We counted as CFS those individuals with all four of the above symptoms, each lasting for at least 6 months. Our definition of at least four symptoms meets the threshold criteria, as CFS is usually diagnosed on the basis of four of eight symptoms, including fatigue ." It is incorrect to say: "CFS is usually diagnosed on the basis of four of eight symptoms, including fatigue" One can either say: "CFS is usually diagnosed on the basis of *five* of nine symptoms, one of which must be fatigue" Or "CFS is usually diagnosed on the basis of *four* out of eight symptoms, *plus* fatigue". This doesn't appear to be a typo: the questions used only involve 4 symptoms *including* fatigue: "The questions asked about the following symptoms: (1) getting tired and lacking energy and whether there were any reasons for this (such as physical illness), (2) having any problems with concentrating or noticing any problems with forgetting things, (3) having problems with falling asleep or with getting back to sleep, (4) any sort of ache or pain or being troubled by any sort of discomfort such as headache or indigestion." The symptoms that make up the referenced definition are: Fatigue Plus 4 out of 8 of the following: "post-exertional malaise lasting more than 24 hours; unrefreshing sleep; impaired short-term memory or concentration severe enough to cause substantial reduction in previous levels of occupational, educational, social, or personal activities; headaches of a new type, pattern, or severity; muscle pain; multi-joint pain without swelling or redness; sore throat; and tender cervical/axillary lymph nodes." The authors explain that what they is discussed is CFS-like illness as the individuals were not assessed individually by a physician to check for exclusions. The authors state they “were able to identify diseases that might exclude a diagnosis of CFS (31 of 108 (28.7%) participants met the criteria for CFS): cancer, diabetes, epilepsy, arthritis or fibrositis and infectious or parasitic disease.” We probably do not have enough data from previous work to estimate the percentage who would have been excluded from a diagnosis of CFS following an individual assessment; however one large expensive US study conducted by the CDC  which used a comparable definition  to the 2003 definition found that while 1.607% had a CFS-like illness, only 0.235% had CFS (or 14.62% of those with a CFS-like illness). If the figure was similar for the current study, only 16 out of the 108 would have CFS. While not evidence per se about the validity of the CFS definition used, the fact that female gender was not a risk factor for the "diagnosis" in this study adds another level of doubt about the cohort of "CFS" patients used, given the consistent findings that CFS is more prevalent in female adults e.g. [4,5]. The NICE guidelines for "CFS/ME" say it affects "women at four times the rate of men". Finally, I think there is a risk that the "risk factors" mentioned in the abstract will be misread by some: an ongoing discussion in the CFS field is whether a certain level of activity predisposes one to CFS e.g. whether people with CFS were more or less active than the general population before becoming ill [7-13]. However, this study does not attempt to look at the period before individuals become ill; instead, it makes use of data collected close to when the symptoms were assessed, using a cross-sectional type of design; it is not thus not that surprising that such individuals [with CFS] were found to relatively inactive given the definition of CFS. References:  Bhui KS, Dinos S, Ashby D, Nazroo J, Wessely S, White PD. Chronic fatigue syndrome in an ethnically diverse population: the influence of psychosocial adversity and physical inactivity. BMC Med. 2011 Mar 21;9(1):26.  Reeves WC, Lloyd A, Vernon SD, Klimas N, Jason LA, Bleijenberg G, Evengard B, White PD, Nisenbaum R, Unger ER, International Chronic Fatigue Syndrome Study Group: Identification of ambiguities in the 1994 chronic fatigue syndrome research case definition and recommendations for resolution. BMC Health Serv Res 2003, 3:25.  Reyes M, Nisenbaum R, Hoaglin DC, Unger ER, Emmons C, Randall B, Stewart JA, Abbey S, Jones JF, Gantz N, Minden S, Reeves WC (2003). Prevalence and incidence of chronic fatigue syndrome in Wichita, Kansas. Archives of Internal Medicine 163, 1530–1536.  Fukuda K, Straus SE, Hickie I, Sharpe MC, Dobbins JG, Komaroff AL, and the International CFS Study Group (1994). The chronic fatigue syndrome: a comprehensive approach to its definition and study. Annals of Internal Medicine 121, 953–959.  Jason LA, Richman JA, Rademaker AW, Jordan KM, Plioplys AV, Taylor RR, McCready W, Huang C-F, Plioplys S (1999). A community-based study of chronic fatigue syndrome. Archives of Internal Medicine 159, 2129–2137.  Turnbull N, Shaw EJ, Baker R, Dunsdon S, Costin N, Britton G, Kuntze S and Norman R (2007). Chronic fatigue syndrome/myalgic encephalomyelitis (or encephalopathy): diagnosis and management of chronic fatigue syndrome/myalgic encephalomyelitis (or encephalopathy) in adults and children. London: Royal College of General Practitioners.  Smith WR, White PD, Buchwald D: A case control study of premorbid and currently reported physical activity levels in chronic fatigue syndrome. BMC Psychiatry 2006, 6:53.  Harvey SB, Wadsworth M, Wessely S, Hotopf M: Etiology of Chronic Fatigue Syndrome: Testing Popular Hypotheses Using a National Birth Cohort Study. Psychosom Med. 2008 Mar 31  Viner R, Hotopf M: Childhood predictors of self reported chronic fatigue syndrome/myalgic encephalomyelitis in adults: national birth cohort study. BMJ 2004, 329:941.  Riley MS, O'Brien CJ, McCluskey DR, Bell NP, Nicholls DP: Aerobic work capacity in patients with chronic fatigue syndrome. BMJ 1990, 301:953-6.  Van Houdenhove B, Onghena P, Neerinckx E, Hellin J: Does high "action-proneness" make people more vulnerable to chronic fatigue syndrome? A controlled psychometric study. J Psychosom Res 1995, 39:633-40.  MacDonald KL, Osterholm MT, LeDell KH, White KE, Schenck CH, Chao CC, Persing DH, Johnson RC, Barker JM, Peterson PK: A case-control study to assess possible triggers and cofactors in chronic fatigue syndrome. Am J Med 1996, 100:548-54.  Van Houdenhove B, Neerinckx E, Onghena P, Lysens R, Vertommnen H: Premorbid "overactive" lifestyle in chronic fatigue syndrome and fibromyalgia: an etiological relationship or proof of good citizenship? J Psychosom Res 2001, 51:571-6. Competing interests I am information officer with the Irish ME/CFS Association - for Information, Support & Research. All my work for the organisation is voluntary (i.e. unpaid).