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The worldwide pandemic of obesity is creating unique challenges for the diagnosis and treatment of asthma. A wealth of epidemiologic literature has established that whereas asthma can lead to obesity, obesity is a risk factor for asthma, but mechanisms are unclear. This review assesses the current understanding of the relationship between obesity and asthma.Recent studies are developing a more sophisticated understanding of the possible inflammatory, immunologic, genetic, and mechanical mechanisms underlying the association between obesity and asthma. Obese asthma may be a unique phenotype of asthma, with a more difficult clinical course and altered response to asthma controller therapy. Adipokines such as leptin and adiponectin are thought to be important, but there is new interest in other inflammatory mechanisms related to visceral obesity, insulin resistance, and the metabolic syndrome.There are still far more questions than answers as to how obesity might cause or worsen asthma. It is clear that weight gain and obesity are particularly troublesome in asthmatics, and clinicians should target these individuals for aggressive intervention. Randomized controlled trials are needed to determine the best treatment approaches for obese asthma, and prospective studies in which both obesity and asthma are well characterized are needed to better understand the underlying mechanisms.