Objective: To investigate the relation between skin test reactivity in children and number of siblings. Design: Cross sectional survey among school-children aged 9-11 years. Skin prick tests in the children and self completion of written questionnaire by their parents. Subjects: 5030 children in Munich and 2623 children in Leipzig and Halle, Germany. Main outcome measures: Atopic status assessed by skin prick tests. Results: After possible confounders were controlled for, the prevalence of atopic sensitisation decreased linearly with increasing number of siblings (odds ratio=0.96 for one sibling, 0.67 for five or more siblings; P=0.005). In atopic children the severity of the skin test reaction as assessed by the weal size was not associated with the number of siblings. Conclusions: Factors directly or indirectly related to the number of siblings may decrease the susceptibility of children to become atopic. Thus, declining family size may in part contribute to the increased prevalence of atopic diseases reported in Western countries over the past few decades.
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