In the past decade the use of theophylline was less extensive because of its narrow therapeutically index and due to its side effects. The widening of possibilities of monitoring the treatment by serum level determination, the introduction of slow release formula, and the demonstration of antiinflammatory effects made theophylline to become actual again. Theophylline is indicated for the chronic treatment of asthma in association with other medication. The authors study the efficacy and serum level of theophylline in 13 children with asthma who received slow release theophylline in a dosage of 15.2 mg/body weight/24 hours in average, administered 3 times a day. The efficacy was estimated by a clinical score and the serum level was determined by isotopic mass spectrometry with 15N-theophylline as internal standard. After a period of at least 2 months of therapy it was recorded the amelioration of clinical score (with 5.09 points, that represents 36.3% of the maximum initial score) and the reducing of circadian variation of peak expiratory flow from 27% to 15%. The serum level of theophylline was of 8.25 +/- 4.16 micrograms/ml at 4 hours after administration and of 5.69 +/- 2.6 micrograms/ml at 8 hours after administration of the last dose. The individual values less than 5 micrograms/ml was found in 3 of 13 children at 4 hours and in 7 of 13 children at 8 hours after the last dose. There were no patients with toxic serum levels of theophylline (> 20 micrograms/ml). The correlation between serum level of theophylline and the dosage was weak both at 4 hours (r = 0.054) and 8 hours (r = 0.229) after the last dose. At the same dose there were found high interindividual variations. These findings are arguments for the usefulness of determination of serum levels of theophylline, that together with clinical and functional parameters allowed the individualization of the dosage.