Sex differences in asthma prevalence and morbidity, assessed with different methods in different populations, have raised several hypotheses about the different susceptibility to asthma in men and women. However, information on the incidence of asthma by age and sex is limited. The aim of this study was to estimate the age- and sex-specific incidence of asthma from birth to 44 yr of age in men and women across several countries, and to evaluate the main factors influencing asthma incidence in young adults. The data of the European Community Respiratory Health Survey, an international, cross-sectional, population-based survey, which were collected in 16 countries from 1991 to 1993 according to a common protocol, and which pertained to 18,659 subjects, were analyzed retrospectively, using the reported age of the first attack as the onset of asthma. During childhood, girls had a significantly lower risk of developing asthma than did boys (relative risk [RR]: 0.74 and 0.56 in the 0- to 5-yr and 5- to 10-yr age classes, respectively). Around puberty, the risk was almost equal in the two sexes (RR = 0.84; 95% confidence interval [CI]: 0.65 to 1.10 in the 10 to 15-yr age class). After puberty, the risk in women was always significantly higher than that in men (RR: 1.38 to 5.91). This pattern was consistent in all of the 16 countries studied, and was not influenced by recall or cohort effects. When the effects of airway caliber and smoking were studied with a case-control design, the results showed that women's greater susceptibility to asthma in early adulthood was at least partly, explained by their smaller airway caliber (the OR decreased from 2. 04 [95% CI: 1.32 to 3.15] to 1.47 [95% CI: 0.89 to 2.44] after controlling for height-adjusted FEV(1)); while smoking did not increase the risk. This analysis strongly confirms that the incidence of asthma shows a sex reversal during puberty, and suggests that airway caliber, in addition to hormonal factors, could play an important role in explaining the different patterns of asthma incidence in men and women.