Asthmatic patients during sympton-free periods almost invariably have abnormalities in lung mechanics and gas exchange. Tentacious secretions and mucosal thickening exaggerate maldistribution of ventilation and cause flow limitation in small airways. Hence, the maximal expiratory flow volume loop in these patients will show impaired flow rates at low lung volumes and many will show a widened alveolar-arterial O2 tension difference. Preventive treatment should be aimed at reversing these abnormalities. The regular use of inhaled sympathomimetics and oral theophylline preparations is justified in the symptom-free patient whose history suggests that he is susceptible to acute exacerbations. Such patients commonly experience an improved sense of well-being, increased exercise tolerance, and a decrease in the frequency and severity of their acute episodes.