Asthma and obesity have a considerable impact on public health and their prevalence has increased in recent years. Numerous studies have linked these disorders. Most prospective studies show that obesity is a risk factor for asthma and have found a positive correlation between baseline body mass index and the subsequent development of asthma. Furthermore, several studies suggest that whereas weight gain increases the risk of asthma, weight loss improves the course of the illness. Different factors could explain this association. Obesity is capable of reducing pulmonary compliance, lung volumes, and the diameter of peripheral respiratory airways as well as affecting the volume of blood in the lungs and the ventilation-perfusion relationship. Furthermore, the increase in the normal functioning of adipose tissue in obese subjects leads to a systemic proinflammatory state, which produces a rise in the serum concentrations of several cytokines, the soluble fractions of their receptors, and chemokines. Many of these mediators are synthesized and secreted by cells from adipose tissue and receive the generic name of adipokines, including IL-6, IL-10, eotaxin, tumor necrosis factor-alpha, transforming growth factors-beta1, C-reactive protein, leptin, and adiponectin. Finally, specific regions of the human genome related to both asthma and obesity have been identified. Most studies point out that obesity is capable of increasing the prevalence and incidence of asthma, although this effect appears to be modest. The treatment of obese asthmatics must include a weight control program.