BACKGROUND: CD4(+)CD25(+) regulatory T (T(reg)) cells can control the allergic response to allergen, airway eosinophilia and airway hypersensitivity. We speculated that chronic inflammation persisting in asthma airways is dependent on abnormalities of these T(reg) cells. There are differences in the pathology of asthma in adults and children, and the airways of pediatric asthma are considered to be more naive than those of adults. Therefore, we analyzed the functionality of T(reg) cells in pediatric asthma and the relationship between T(reg) function and asthma symptoms. METHODS: The anergic state, which is one of the defining properties of T(reg), was analyzed by measuring intracellular Ca(2+) influx following T cell receptor (TCR) stimulation. FOXP3-positive cells and FOXP3 mRNA expression were measured by flow analysis and real-time PCR with the SYBR method, respectively. RESULTS: CD45RO(+) cells make up approximately 99% of CD4(+)CD25(high) T cells and 89% of CD4(+)CD25(low) T cells in human adult blood. The proportion of CD45RO(+) cells in CD4(+)CD25(+) (high + low) T cells from pediatric asthma was much smaller (about 56%). Interestingly, our data indicated that CD45RO(+) T(reg) cells from pediatric asthma aberrantly increased intracellular Ca(2+) concentrations following TCR activation compared with pediatric nonasthma controls. CONCLUSION: These impaired CD45RO(+) T(reg) cell functions were correlated with asthma symptoms. The correlation was observed in the group with a highly expressed atopic phenotype and longer duration of asthma. We suggest that chronic inflammation in pediatric asthma airways may be the result of impaired regulatory functions of CD45RO(+) T(reg) cells.