Objective To investigate the effect of different doses of vitamin D₃ on T lymphocyte subsets in children with bronchial asthma and on the improvement of asthma control levels,and to explore the mechanism and optimal dose of vitamin D₃ in the treatment of bronchial asthma in children. Methods A total of 120 children with bronchial asthma aged from 6 to 13 years were selected.In addition,30 healthy children were selected as the normal control group in the same period.The selected patients were randomly divided into 4 groups,including one group of conventional treatment and three groups of vitamin D₃ treatment on the basis of conventional treatment.The doses of vitamin D₃ were 400 U,600 U and 800 U per day,respectively,and the total course of treatment was 12 months.The T lymphocyte subsets were detected before treatment,1 month,3 months,6 months and 12 months after treatment.The control level of asthmatic children was counted at 12 months after treatment. Results After 12 months,the levels of CD4,CD4/CD8 in blood were significantly lower than those before treatment,and CD8 was significantly elevated,but compared with the control group,there were still significant differences.After 6 months of treatment,in the 600 U and 800 U treatment groups,the levels of CD4 and CD4/CD8 were decreased more significantly than the other two groups.The asthma control level of the 600 U and 800 U treatment group was significantly higher than that of the other two groups after treatment,but there was no significant difference between the 600 U and 800 U groups (P>0.05). Conclusions Oral intake of different doses of vitamin D₃ can regulate the imbalance of T lymphocyte subsets,which is one of the mechanisms of vitamin D₃ in the treatment of asthma,but it still requires a long process.Vitamin D₃ treatment significantly improved the control level of asthma,daily intake of 600 U may be the best treatment dosage.