Fifty hospitalized children with asthmatic bronchitis and bronchial asthma were treated with a continuous intravenous drip infusion of aminophylline. To investigate the pharmacokinetics of theophylline in the presence of pyrexia, patients were divided into two groups based on body temperature: a pyrexia group (≥ 38°C) and a non-pyrexia group (< 38°C). Theophylline clearance was 0.064 ± 0.017 liters/kg/hr in the non-pyrexia group and 0.049 ± 0.010 liters/kg/hr in the pyrexia group. Theophylline clearance in the non-pyrexia and pyrexia groups was 0.044 ± 0.007 liters/kg/hr and 0.030 ± 0.009 liters/kg/hr (≤ 6 months), 0.071 ± 0.011 liters/kg/hr and 0.047 ± 0.008 liters/kg/hr (6 to ≤ 12 months), 0.084 ± 0.012 liters/kg/hr and 0.055 ± 0.006 liters/ kg/hr (1 to ≤ 2 years), and 0.065 ± 0.007 liters/kg/hr and 0.051 ± 0.001 liters/kg/hr (2 to ≤ 3 years), respectively. In all age groups, theophylline clearance of the pyrexia group was significantly less than that of the non-pyrexia group (p < 0.01), showing that there was a significant pharmacokinetic difference in theophylline clearance between the groups. Multivariate statistical analysis showed that theophylline clearance was affected by pyrexia and age. This study showed that the presence of pyrexia decreases theophylline clearance, and that it affects theophylline clearance in an age-dependent manner. Based on the results of this study, dosages should be designed based on the clearance at the time of pyrexia.