The authors assessed the association between asthma prevalence and socioeconomic status at both the individual and center levels simultaneously.by using data from 32 centers in 15 countries. Included were 10,971 subjects aged 20-44 years selected from the general population and interviewed in 1991-1992. Socioeconomic status at both the individual and aggregated levels was measured on the basis of occupation and educational level. Associations were assessed by using multilevel models adjusted for age, sex, body mass index, parental asthma, childhood respiratory infections, presence of immunoglobulin E to common allergens, rhinitis, smoking, and occupational exposure to irritants. Asthma prevalence was higher in lower socioeconomic groups, whether defined by educational level (odds ratio for finishing full-time studies-<16 vs. >19 years = 1.28, 95% confidence interval: 1.00, 1.64) or social class (odds ratio for semiskilled and unskilled manual workers vs. professional/managerial = 1.51, 95% confidence interval: 1.20, 1.90), regardless of atopic status. The relation was consistent between centers. Irrespective of individual socioeconomic status, subjects living in areas in which educational levels were lower had a higher risk of asthma (p < 0.05). This center-level association partially explained geographic differences in asthma prevalence, but considerable heterogeneity still remained. The authors concluded that community influences of living in a low-educational area are associated with asthma, independently of subjects' own educational level and social class.