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Epidemiological studies first demonstrated the association between obesity and asthma and they have begun to provide additional evidence to support causality: a dose-effect relationship, consistency across studies (especially among women) and the correct temporal order (ie, obesity before asthma). To date, relatively few studies have addressed reversibility, an important but less frequently demonstrated epidemiological criterion of causality. Reversibility suggests that if excessive weight is a risk factor for asthma, then reducing body weight should decrease the prevalence of asthma, or at least decrease asthma related symptoms or health care utilisation. We performed a systematic review on weight loss and asthma, based on searches between January 1966 and January 2007 of both PubMed and the Cochrane Clinical Trial Database. Of the 15 relevant studies, asthma was the primary outcome in only five. Only one study was conducted in children. Regardless of the type of intervention (surgical vs medical), all 15 studies noted an improvement in at least one asthma outcome after weight loss. The improvement was noted across studies that differed in sample age, gender or country of origin. The heterogeneity of the interventions and outcomes precluded quantitative synthesis. We briefly review the role of specific factors (eg, gastro-oesophageal reflux) in the weight loss-asthma association, and potential directions for future research.