Objective: Asthma and obesity are among the major causes of morbidity in childhood and adolescence. Early obesity increases the chances of chronic degenerative diseases in adults. Although the concomitance or both clinical situations are being demonstrated in various studies, the intrinsic mechanisms of this association are still very little known. Therefore, the objective of this article was to review the main studies on the association of obesity and asthma and check if there is a cause-effect relation between them. Sources: Systematic review based on indexed data bases MEDLINE (PubMed) and SciELO. Original articles (cross-sectional, case-control, and prospective studies) and meta-analysis published in the period that ranges from January 1998 to January 2008 were reviewed. Studies published in English, Spanish, and Portuguese were researched. Summary of the findings: Although there are various studies on growing prevalence of asthma and obesity, few of them establish cause-effect relations between them. Physiopathological mechanisms and factors involved in this process are still little known. Conclusion: Methodological rigor in future studies must seek for answers to better understand if there is association between asthma and obesity or if the relationship between both diseases is a coincidence. Copyright © 2010 by Sociedade Brasileira de Pediatria.861614[IV Brazilian Guidelines for the Management of Asthma]. J Bras Pneumol. 2006;32 Suppl 7:S447-74Tantisira KG, Weiss ST. Complex interactions in complex traits: obesity and asthma. 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