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Background and Purpose: The aim of this study was to investigate the frequency of CD4(+)CD25(+) regulatory T cells and their phenotypic expression in peripheral blood of children with active and non-active juvenile idiopathic arthritis (JIA) and healthy controls, to determine if their frequency or phenotypic expression is involved in the immunoregulation of this disease., Methods: From October 2004 to October 2005, 55 JIA patients and 55 age- and gender-matched healthy controls were enrolled in the study at National Taiwan University Hospital. Flow cytometry was used to determine the frequency of CD4(+)CD25(+) and CD4(+)CD25(hi) in CD4(+) T cells and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) expression on CD4(+)CD25(+) by tricolor staining. Basic profiles, medication history, clinical symptoms and laboratory data were obtained by chart review and outpatient department interviews., Results: There was no significant difference in expression of CD4(+)CD25(+) T cells between patients with inactive JIA and normal controls (13.74+/-3.25% vs 12.85+/-3.68%, p>0.05). The expression of CD4(+)CD25(hi) T cells was significantly lower in inactive JIA patients than in normal controls (1.89+/-1.01% vs 2.76+/-1.28%, p<0.01). The expression of CTLA-4 on CD4(+)CD25(+) T cells was also significantly lower in inactive JIA patients compared with controls (4.37+/-2.02% vs 6.33+/-2.57%, p<0.001)., Conclusion: We speculate that a decreased frequency of CD4(+)CD25(hi) regulatory T cells and lower level of CTLA-4 expression on CD4(+)CD25(+) regulatory T cells might play a role in the immunoregulation of JIA.