In an open study 19 patients with chronic obstructive lung disease were treated over nine days with Afonilum retard (1 capsule contains 250 mg pure theophylline in retard pellets form, the mite-form contains 125 mg pure theophylline in retard pellets form). Vital capacity (VC), forced expiratory volume (FEV1) and bronchial resistance (Rt) were measured before and on the third, sixth, and ninth day at eight o'clock and four hours later. Theophylline serum levels were measured by radioimmunoassay. In 14 patients 2 capsules (250 mg theophylline twice a day) were administered in twelve hours intervals. In this group lung function improved statistically significantly (p less than 0.01) for VC to 37%, FEV1 to 80% and Rt to 64% in comparison to the values before. Mean values of theophylline serum concentrations were 6.9 +/- 2.1 micrograms/ml at eight o'clock before the morning dose and 9.6 +/- 2.2 micrograms/ml four hours later on the ninth day. In five patients 375 mg theophylline were administered twice a day from third day on. In this group lung function improved for VC to 50%, FEV1 to 75% and Rt to 61% compared with the values before. Mean values of theophylline serum concentrations resulted in 10.8 +/- 3.6 micrograms/ml at eight o'clock before the morning dose and in 15.0 +/- 3.5 four hours later on the ninth day.