Obesity and asthma are two disorders of high and increasing worldwide prevalence. A consistent association between obesity and asthma has been recently found in case-control, cross-sectional and longitudinal studies. This association is more consistent in women after the puberty. Moreover, an improvement in the severity of asthma has been described after weight reduction in obese patients. A causal association between asthma and obesity may represent an additional respiratory work that could increase the morbidity and medical expenditures. The most striking studies which demonstrate association between obesity and asthma and the possible causal mechanisms are reviewed.