Background. Chronic fatigue syndrome (CFS) remains an idiopathic and controversial entity.
Method. We screened 31405 individual members of the Swedish Twin Registry (aged 42–64 years) for the symptoms of fatiguing illness via a telephone questionnaire. We refined self-reported symptoms via data from several national registries and from physician review of all available medical records in order to approximate closely the dominant case definition of CFS.
Findings. The 6-month prevalence of CFS-like illness was 2·36% (95% CI 2·19–2·53) and was markedly higher in women than men, odds ratio 3.92 (95% CI 3·24–4·72) with no significant association with age or years of education. There was a highly significant association with occupation that disappeared after accounting for gender.
Interpretation. CFS-like illness may be more common that previously acknowledged. There is a marked increase in risk by gender. Previous reports that CFS is more prevalent in individuals in certain occupational categories were not confirmed and may have been due to confounding by gender.
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