Theophylline concentrations in serum between 10 to 20 micrograms/ml cannot be obtained with usual dosage of theophylline in children with individual very short elimination half time of theophylline. Multiple corrections of dosage and repeated controls of theophylline concentrations in serum are necessary for empirical adjustment of individual optimal dosage. Pharmacokinetic investigations were made to shorten this management. 12 patients with bronchial asthma, 6 to 18 years of age, received 1.5 to 3 tablets of Aminophyllin under conditions of steady state. Blood samples were taken up to 6 hours after oral application. As a result of statistical analysis we found that the biological half time of theophylline could be determined from 2 to 3 blood samples. Estimation of optimal individual dosage is possible by simple mathematical pharmacokinetic formulas.