We compared serum theophylline concentrations in patients treated with one of two commercially available theophylline preparations: a sustained-release aminophylline and a sustained-release theophylline. Two comparable groups of 15 out-patients with stable, chronic obstructive lung diseases were studied: one group was given sustained-release aminophylline while the other took sustained-release theophylline. Both drugs were administered orally for 7 days at a daily dose, equivalent to 12 mg/kg in terms of anhydrous theophylline. Serum theophylline concentrations were always significantly lower after treatment with sustained-release aminophylline than after treatment with sustained-release theophylline, which latter frequently caused undesirable side-effects. Moreover, patients receiving sustained-release aminophylline always showed serum theophylline concentrations lower than 10 mcg/ml. Pulmonary function tests were unaffected by the administration of either drug. We conclude that sustained-release theophylline is more effective than sustained-release aminophylline in terms of induced serum theophylline concentrations. However neither drug was suitable for the treatment of patients with chronic obstructive lung disease without other concomitant therapy.