There is increasing evidence linking Western culture to increasing prevalence of asthma. We have measured the prevalence of symptoms of asthma, bronchial hyperreactivity, and atopy in a sample of white and Asian inner city schoolchildren aged 8-11 yr in the U.K. and assessed the influence of certain aspects of western culture on these variables. Bronchial reactivity was measured by methacholine challenge, atopy by allergen skin prick tests, and symptoms of asthma and aspects of lifestyle by questionnaire. Degree of Westernization was assessed from dietary habit categorized as exclusively Asian, mostly Asian, mixed, or exclusively English. The questionnaire was completed by 92% of children. The 308 white and 539 Asian children were of similar age (mean 9.3 yr versus 9.4 yr) with slightly more girls in the Asian group (52.6% versus 47.4%). The white children reported more wheeze within the past year (14.2% versus 8.8%, p = 0.01) but less were atopic (29.5% versus 38.4%, p = 0.01) with no significant difference in the prevalence of hyperreactivity (31.5% versus 29.6%, p = 0.57). Most Asians (97%) included Asian food in their diet; 35% ate a mostly Asian diet, 45% ate a mixed English and Asian diet, and 17% ate an exclusively Asian diet. After controlling for confounding factors whites were at higher risk of hyperreactivity and lower risk of atopy, and the risk of hyperreactivity and to a lesser extent atopy among Asian children was related to diet in a dose response fashion. The relative odds for hyperreactivity with an exclusively Asian, mostly Asian, or a mixed diet relative to an English diet were 0.31 (95% CI 0.15 to 0.62), 0.88 (0.56 to 1.37), and 0.99 (0.65 to 1.49), and for atopy were 1.22 (0.72 to 2.06), 1.47 (0.99 to 2.19), and 1.78 (1.23 to 2.58). These results suggest that diet or other closely related factors associated with Westernization are involved in the etiology of atopy and hyperreactivity.