The bronchodilator, respiratory center and respiratory muscle effects of oral sustained release theophylline are investigated with a placebo controlled double-blind study in 34 patients with chronic obstructive lung disease (COLD). The first 3 days were a theophylline washout period and the patients did not receive theophylline and on the 4th day, serum theophylline concentration was negligible. Initial pulmonary function tests were performed. Twenty-four of the patients took 350 mg oral sustained release theophylline twice a day. Ten of the patients were given placebo. On day 7, serum theophylline concentrations of the study group were in therapeutic doses and the tests were repeated. Bronchodilator effect was assessed by spirometry, flow-volume loops and measuring airway resistance (Raw). Respiratory muscle function was assessed by measuring maximal inspiratory (PImax) and expiratory pressures (PEmax) and the effect on respiratory center was evaluated by minute ventilation, mouth occlusion pressure index and inspiratory duty ratio. Forced vital capacity in one second (p less than 0.001), maximal mid expiratory flow rate (p less than 0.05), maximal expiratory flow at 50 percent of vital capacity (p less than 0.001), maximal inspiratory pressure (p less than 0.01) increased, while airway resistance (p less than 0.05) decreased significantly. Mouth occlusion pressure and minute ventilation mouth occlusion pressure index did not change after theophylline therapy. In placebo group, there was no significant change in airway obstruction assessed by spirometry and flow volume loops or in respiratory muscle function assessed by maximal respiratory pressures. It is therefore, concluded that oral sustained release theophylline has a bronchodilator effect in irreversible chronic obstructive lung disease, increases respiratory muscle function, but in therapeutic doses has no effect on respiratory center.