In 9 "stable" adult asthmatics, the relationship between serum theophylline level and pulmonary function, as well as the interaction of aminophylline and inhaled beta2-agonist on pulmonary function, were studied. At varying levels of serum theophylline between 6 and 25 micrograms/ml, which were brought about by increasing oral doses of a slow-release theophylline preparation, the forced vital capacity (FVC), forced expiratory volume in one second (FEV1), airway resistance (Raw), and lung volume were assessed before and after inhalation of a B2-sympathicomimetic (isoetharine). In those subjects in whom it was possible to obtain blood levels within all 3 ranges of less than 10, 11 to 15, and greater than 15 micrograms/ml, there was a significant (p less than 0.05) increase in FEV1 between a mean level of 6.4 +/- 0.9 micrograms/ml and one of 12.8 +/- 0.2 micrograms/ml, but there was no further significant improvement at a mean value of 19.2 micrograms/ml +/- micrograms/ml. Specific airways conductance (SGaw) showed no change at increasing serum theophylline levels. In addition, inhaled beta-agonist increased FEV1 and SGaw maximally at all times, irrespective of the serum theophylline level, and there was no relationship between the response to inhaled bronchodilator and the serum theophylline level. The data indicate that unlike the patient in serious difficulty, ventilatory function was not improved by maximizing serum theophylline levels in this population of stable asthmatics. This suggests that theophylline may be less useful for the stable asthmatic patient than for the acutely ill patient.