25 patients suffering from chronic obstructive lung disease with a delta FEV1 greater than 15% were included in an open uncontrolled study. Only 17 of these patients participated during the entire length of the study, for there were 8 dropouts (five because of side effects). 13 of these 17 patients had been treated with Euphylong (2 capsules a 375 mg theophylline every evening, i.e. a daily dose of 750 mg theophylline once every evening.) 4 patients were given additionally another capsule of Euphylong in the morning because their theophylline requirements were higher. After this dose titration, the patients were continually treated for another 5 days; on the last day, a 24-hour profile of the peak flow and of serum theophylline concentration was recorded at 2-hour intervals. The course of serum theophylline concentration showed a typical "plateau" profile in all the patients, the maximum values being attained in the night and during early morning hours. The maximum value Cmax of 13.7 +/- 1.3 or 14.4 +/- 3.1 mg/l occurred on the average 8.1 +/- 1.1 or 7.5 +/- 0.9 hours after intake in the evening (the first-named value at once-daily dosage, the second at twice-daily dosage). Fluctuation at single dosage was 82 +/- 9% and at twice daily dosage 44 +/- 10%. the average daily level (CAV) was in the therapeutic range with both dosage schemes (10.1 +/- 0.9 or 12.0 +/- 2.5 mg/l). There was no morning dip in the early morning hours. Because of the observed side effects that are typical of theophylline, we recommend to administer Euphylong in the first phase of therapy initially in small doses gradually increasing to the desired maximum, in accordance with the procedure usually followed in optimised theophylline treatment courses.