Theophylline is commonly prescribed for patients with nonasthmatic chronic airflow obstruction (CAO) even though clinical efficacy is not well established. We studied objective and subjective responses to theophylline in 14 men with CAO. Subjects randomly received week-long treatments of placebo or theophylline at two dosages: one that produced low (8.7-13.0 μg/ml) and the other high (16.0-23.6 μg/ml) plasma concentrations. During the final three days of each treatment, we measured spirometric and hemodynamic function. Exercise tolerance was assessed with the 12 minute walk and progressive cycle ergometry. The patients' perception of breathlessness during the usual activities of daily living was evaluated with the oxygen cost diagram and the breathlessness rating. For low and high dose theophylline there were significant (p < .05) increases in forced vital capacity (7.1 ± 2.1 percent; 12.0 ± 1.7 percent), forced expiratory volume at one second (14.6 ± 4.9 percent; 12.1 ± 3.3 percent) and in pulse rate (8.3 ± 1.2 percent; 19.1 ± 3.1 percent), but no changes in blood pressure. There were also no significant differences among the three treatments for any of the tests which assessed exercise tolerance or breathlessness. These results suggest that most patients with CAO experience little symptomatic benefit from taking theophylline.