Asthma and other respiratory diseases have increased in the last years among Venezuelan children from helminthic endemic areas where the infection by Ascaris lumbricoides has been associated to bronchial airway inflammation in parasitized individuals. The aim of this work was to investigate the possible associations between the development of bronchial hyper reactivity and the immune response against A. lumbricoides in urban and rural children.We evaluated 470 school children from rural and urban communities. Pulmonary function tests were performed and ≥20% PC20 changes were considered as a positive diagnostic of bronchial hyper reactivity. The prevalence and intensity of A. lumbricoides infectionwas determined by faecal examination. Specific serum IgE levels using a modified ELISA and skin prick tests against A. lumbricoides and the common allergen Dermatophagoides pteronyssinus were done. The number of circulating lymphocyte sub populations was determined by flow cytometry analysis. In rural children, bronchial hyper reactivity was associated with increased specific levels of anti-A. lumbricoides IgE (p < 0.0001) and skin test positivity for A. lumbricoides (p < 0.0001). The percentage of FEV1 predictive values correlated inversely (p < 0.0001) with anti-A. lumbricoides IgE levels. Elevated numbers of circulating CD3+CD4+ and CD20+CD23+ cells were found in rural children with bronchial hyper reactivity compared to their asymptomatic counterparts. They correlated positively with anti-A. lumbricoides IgE levels (p < 0.005 and < 0.0001, respectively). In contrast, in urban children, bronchial hyper reactivity was associated with elevated anti-D. pteronyssinus IgE levels (p = 0. 0089), skin hyper reactivity towards this aero allergen (p = 0.003) and to an increase in the number of CD3+CD8+ (p < 0.0001). Our results suggest that the IgE response against A. lumbricoides infection may be involved in the development of bronchial hyper reactivity among rural children from endemic areas and also that improved hygienic conditions in the urban environment is associated with increased responses to airborne allergens. © 2007 Elsevier B.V. All rights reserved. Keywords: Ascaris infection; Bronchial hyper reactivity; IgE antibodies; CD23 receptorEste trabajo demuestra la relacion entre la presencia de antigenos de Ascaris lumbricoides con la hiperreactividad bronquial presente en la poblacion infantil rural Venezolana.Projects FONACIT: G2005000371 and G2005000374.