OBJECTIVE: To study the changes of serum interleukin-15 (IL-15) levels and the expression of CD4+T (T-helper lymphocyte) subsets CD4+CD45RA+ and CD+CD45RO+ in peripheral blood of children with juvenile rheumatoid arthritis (JRA). METHODS: The serum concentration of IL-15 was detected using ELISA in 39 children with JRA. The expressions of CD4+CD45RA+T and CD4+CD45RO+ T in peripheral blood were detected by flow cytometry in 24 out of the 39 patients with JRA. Twenty-six age and sex-matched healthy children were used as the Control group. RESULTS: The mean serum IL-15 level in JRA patients was significantly higher than that in controls (1.37±0.98 pg/mL vs 0.96±0.41 pg/mL, P<0.05). Among the 39 JRA patients, the serum IL-15 level in 17 patients with systemic JRA increased remarkably (P<0.01), but not in patients with the other two types of JRA, the oligoarthritis and polyarthritis (n=13, n=9, respectively), compared with that in controls. The mean serum IL-15 level of the JRA patients was significantly reduced after conventional treatment (P<0.01). The serum IL-15 level in JRA patients positively correlated with white blood cell count (r=0.347, P<0.05) and C reactive protein (r=0.452, P<0.01), but not with the erythrocyte sedimentation rate. The patients with high serum IL-15 levels (≥medium level 1.73 ρg/mL) had higher expression of CD4+CD45RO+T than those with low serum IL-15 levels