There is little doubt that the cause of the increased prevalence and severity of asthma is multifactorial. Although the factors of allergen exposure and hygiene are almost certainly necessary for its development, there is a growing body of literature that implicates lifestyle change, specifically decreased physical activity, as a contributor to the increase in asthma prevalence and severity. Several literature reviews of exercise conditioning in patients with asthma have been published. These reviews and recent controlled trials emphasize that although many of the studies of exercise conditioning in asthmatic patients involved different methods and outcome measures, the overwhelming majority of studies demonstrated the capacity for asthmatic subjects to exercise safely and significantly improve their cardiovascular fitness and quality of life. There are several proposed pathophysiologic mechanisms responsible for the effects of decreased activity on the lung function of patients with asthma. A prescription for exercise has been endorsed for all asthmatic subjects by the American College of Sports Medicine and the American Thoracic Society. The allergy community has placed emphasis on medical therapy and allergen avoidance; in addition, exercise [correction] has not been formally incorporated into the National Asthma Education and Prevention Program guidelines. It is our belief that an exercise prescription should be part of the treatment for all cases of asthma. The real question is whether prolonged physical activity and, in particular, outdoor play of children plays a role in prophylaxis against persistent wheezing. If so, the decrease in physical activity might have played a major role in recent increases in asthma prevalence and severity.