Background-The timing and mechanism of the inverse association between increasing sibling number and atopic disease are not yet understood. A study was undertaken to examine how family size at birth predicts early respiratory illness, to report the association between infant respiratory illness and childhood atopic disease, and to determine whether the protective eVect of large family size operates during infancy or later childhood.

Objectives-To document the relation between sibling number and atopic disease, and to assess the contribution of possible confounding factors to the protective eVect of siblings in relation to asthma and hay fever. Design and subjects-Cross sectional survey by parental questionnaire in Tasmania, Australia, on 6378 children (92% of those eligible) who reached 7 years of age during 1995. Methods-Exercise challenge lung function testing was conducted on 428 children. Analyses reported were conducted on singleton births only (n = 6158). Results-The prevalences of a history of asthma ever, hay fever, and eczema were 27%, 19%, and 22%, respectively. Asthma and hay fever, but not eczema, were inversely related to sibling number, with evidence of a dose-response trend. The mean age at onset for asthma or wheezy breathing decreased as the number of siblings increased. The inverse association between sibling number and asthma or hay fever persisted after adjustment for several confounders, such as parental smoking or breast feeding, but did not persist after adjustment for household size in 1995. Conclusions-The protective eVect of high sibling number could not be separated from household size at age 7, and it appears to be operating after birth and influences the age at onset of asthma symptoms. Further work to increase knowledge of how the protective eVect of the presence of siblings works might have important implications for the understanding of the pathogenesis of asthma. (Arch Dis Child 1998;79:328-333)