None of the explanations proposed for the increase in paediatric asthma have been adequate. It is becoming apparent that the cause of the increase in asthma must be multi-factorial. Increasing attention has been focused on the role of lifestyle in the development of asthma. Lifestyle changes that have occurred in children are those in diet and decreased physical activity, with obesity being the product of these changes. The increase in asthma, obesity and a sedentary lifestyle have occurred together. However, a temporal relationship between asthma, obesity and decreased physical activity has not been determined in the paediatric literature. Limited data suggest that decreased physical activity could be playing a role in the aetiology of asthma independent of obesity. Furthermore, there has been substantial research on the benefits of exercise programmes for paediatric patients with asthma. Longitudinal trials monitoring physical activity, obesity and the development of asthma are needed.