The kinetics and the effects on the ventilatory function peak expiratory flow rate (PEFR) of a single 600-mg oral dose of theophylline were investigated in 46 adult patients with bronchial asthma (BA) and in 16 adult patients with chronic airflow obstruction (CAO). In the former, theophylline induced an early and potent bronchodilatation (60% rise in PEFR), the kinetics of which correlated with plasma concentration. Theophylline was also effective in patients with CAO, but the magnitude of its bronchodilator effect was less than in those with BA: this was despite plasma concentrations of much the same order. In adult patients with BA (but not with CAO) theophylline plasma levels and bioavailability are higher after simultaneous erythromycin dosing.