BACKGROUND The literature describing the global prevalence of anxiety disorders is highly variable. A systematic review and meta-regression were undertaken to estimate the prevalence of anxiety disorders and to identify factors that may influence these estimates. The findings will inform the new Global Burden of Disease study. Method A systematic review identified prevalence studies of anxiety disorders published between 1980 and 2009. Electronic databases, reference lists, review articles and monographs were searched and experts then contacted to identify missing studies. Substantive and methodological factors associated with inter-study variability were identified through meta-regression analyses and the global prevalence of anxiety disorders was calculated adjusting for study methodology.   
 RESULTS The prevalence of anxiety disorders was obtained from 87 studies across 44 countries. Estimates of current prevalence ranged between 0.9% and 28.3% and past-year prevalence between 2.4% and 29.8%. Substantive factors including gender, age, culture, conflict and economic status, and urbanicity accounted for the greatest proportion of variability. Methodological factors in the final multivariate model (prevalence period, number of disorders and diagnostic instrument) explained an additional 13% of variance between studies. The global current prevalence of anxiety disorders adjusted for methodological differences was 7.3% (4.8-10.9%) and ranged from 5.3% (3.5-8.1%) in African cultures to 10.4% (7.0-15.5%) in Euro/Anglo cultures.   
 CONCLUSIONS Anxiety disorders are common and the substantive and methodological factors identified here explain much of the variability in prevalence estimates. Specific attention should be paid to cultural differences in responses to survey instruments for anxiety disorders.
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