Weinmayr G, Weiland SK, Björkstén B, et al. Am J Respir Crit Care Med. 2007;176(6):565–574 PURPOSE OF THE STUDY. The International Study of Asthma and Allergies in Childhood (ISAAC I) found that the prevalence of asthma symptoms varies >15-fold among various countries. ISAAC II was designed to identify why such differences occur. STUDY POPULATION. Studied were a random sample of 8- to 12-year-old children (n = 54 439) in 22 countries worldwide. METHODS. Data were collected by parental questionnaires (n = 54 439) and skin-prick tests (n = 31 759). Economic development was assessed by gross national income per capita. RESULTS. Prevalence of current wheeze (wheeze during the last year) ranged from 0.8% (Ecuador) to 25.6% (Brazil). The prevalence of skin-prick test reactivity ranged from 1.7% (Ghana) to 45.3% (Hong Kong). The association between current wheeze and skin-prick test reactivity was stronger in affluent countries (odds ratio: 4.0 [95% confidence interval: 3.5–4.6]) than nonaffluent countries (odds ratio: 2.2 [95% confidence interval: 1.5–3.3]). The population attributable fraction (PAF), or fraction of current wheeze attributable to skin-prick test reactivity, ranged from 0% (Turkey) to 59.6% (Hong Kong). Overall, the combined PAFs were substantially higher in affluent countries (40.7%) than in nonaffluent countries (20.3%). CONCLUSIONS. The authors concluded that the link between atopic sensitization and asthma symptoms in children differs strongly between populations and increases with economic development. REVIEWER COMMENTS. The variation in rates of asthma and allergy around the world are striking, and the fact that the relationship between allergy and asthma seems related to the level of economic development in any given country is fascinating. The authors speculated that this may be a result of exposures or other factors that are different for children in more or less affluent countries that make it more or less likely that allergy would lead to asthma. Such factors could include greater exposure to helminth infections or different commensal bacteria in poorer countries and higher rates of urbanization and obesity in wealthier countries.