OBJECTIVE： To study the effects of corticosteroids on CYP3A activity in children at pre-school age. METHODS： 528 healthy children at pre-school age in Xiamen were included in normal children group； 68 children diagnosed as bronchitis were divided into aerosol inhalation group （31 cases） and intravenous administration group （37 cases） according to therapy plan， and they were given budesonide 1 mg， aerosol inhalation， bid or methylprednisolone 1-2 mg， ivgtt， bid. RP-HPLC method was established to determinate the concentrations of 6β-hydroxycortisol and hydrocortisone in urine，and the ratio of 6β-hydroxycortisol and hydrocortisone was used to represent CYP3A activity. Distribution and difference of children CYP3A activity were investigated in 3 groups. RESULTS： The frequency distribution of CYP3A activity in all the 3 groups was a left skewness distribution （P＜0.05， coefficient of skewness＞1）. CYP3A activity of 3 groups were 4.28（1.90，7.69）， 5.80（2.11，11.23） and 7.79（1.93，19.44）respectively， with statistical significance （P＜0.05）. Median CYP3A activity of normal children group was lower than that of intravenous administration， with statistical significance （P＜0.05）. Median CYP3A activity of aerosol inhalation group was higher than normal children group， median CYP3A activity of intravenous administration group was higher than aerosol inhalation group， with no statistical significance （P＞0.05）. CONCLUSIONS： Corticosteroids may influence CYP3A activity in children at pre-school age， and corticosteroids used all over body may change CYP3A activity.