The effect of theophylline on bronchial reactivity and pulmonary mechanics in subjects with chronic lung diseases. The effect of oral administration of theophylline (6 mg/kg) was evaluated in 26 subjects with chronic airflow obstruction and in 20 normal subjects. A forced expiration to at least 75% of vital capacity was used as the index of bronchial reactivity (FEV1.75) and the resistance to airflow as the index of pulmonary mechanics. Neither parameter significantly changed after theophylline. There was no significant difference between the pre and posttheophylline values for any of these parameters in either subject group. Subjects with chronic lung disease have minimal evidence of bronchodilatation following short-term oral administration of theophylline.