Doxophylline, or 2-(7'-theophyllinemethyl)1,3-dioxolane, is a theophylline derivative which has shown interesting bronchodilating activity, and it appears to determine few adverse effects. The aim of the present investigation was to evaluate clinical therapeutic effects of the drug in the treatment of 2 groups of patients suffering from moderate to severe chronic obstructive pulmonary disease differing in acute response to the inhaled beta 2-agonist salbutamol and to compare changes of lung function tests to serum concentration of doxophylline. We studied 67 patients with chronic obstructive pulmonary disease (median age 63 years, 9 females and 58 males) who were all clinically stable at the time of the study. Patients were separated into 2 groups on the basis of their reaction to inhalation of 200 micrograms of salbutamol: those with an increased FEV1 of more than 20% from baseline value (group 1), and those with no increase (group 2). Doxophylline was administered orally at the dose of 400 mg 3 times daily. Serum levels of doxophylline were determined by high-pressure liquid chromatography. Spirometry and blood gas analysis were performed before and 10 days after treatment. Four patients stopped drug assumption because of side effects (3 for dyspepsia and 1 for anxiety). In group 1 (34 patients), a significant increase in SVC, FVC, FEV1, FEF 25-75% and PEFR was observed. In group 1 (29 patients), only PEFR significantly increased. No modifications in blood gas analysis were observed. The mean serum level of doxophylline was 14 micrograms/ml in group 1 and 9 micrograms/ml in group 2: the difference was statistically significant. The relation between serum levels of doxophylline and FVC showed an increase in the parameter up to the concentration of 12-13 micrograms/ml, after which a plateau phase was observed. On the basis of our data, doxophylline appears to have an interesting bronchodilating effect in patients responsive to the inhaled beta 2-agonist salbutamol. The lower limit of the therapeutic range seems to be 12-13 micrograms/ml. The upper limit of the therapeutic range was not determined because it was not possible to obtain serum samples when side effects occurred.