Long-term treatment of childhood asthma with theophylline or theophylline ethylenediamine (Aminophyllin) preparations requires dosage according to the various age groups. In order to achieve therapeutically satisfactory concentrations, estimations of theophylline levels in serum showed that in cases of good correlation between Aminophyllin dosage and theophylline levels smaller children required higher Aminophyllin doses than big children or adults. The therapeutic range of theophylline is around 7 to 15 mg/l beyond the newborn age. In order to obtain these concentrations children from the 1st to the 4th year require Aminophyllin doses of 6 mg/kg t.i.d., children between 5 and 12 years 5.5 mg/kg t.i.d. and adults beyond the 50th year of life 3 to 3.5 mg/kg t.i.d. Generally the first dose should be higher and is 6-7 mg/kg or 6-6.5 mg/kg or 4 mg/kg, respectively. Regular estimations of serum theophylline levels greatly help to improve safety of long-term treatment.