Abstract We examined and evaluated the pharmacokinetics, efficacy, and safety of etanercept in patients with methotrexate (MTX)-refractory polyarticular juvenile idiopathic arthritis (JIA) in Japan. All MTX-refractory polyarticular JIA patients 4 17 years old received 0.4 mg of etanercept per kilogram of body weight subcutaneously twice weekly for up to 3 months in the open-label, prospective, and multicenter trial. A response was defined as an improvement of 30%, 50%, 70%, or more from baseline in at least three of six indicators of disease activity, with no more than one indicator worsening by more than 30% from baseline (30%, 50%, or 70% definition of improvement, respectively), and disease activity score (DAS28) by EULAR (European League Against Rheumatism) response criteria. At the end of the 12-week study, 20 of the 22 patients (90.9%) had responses with both 30% and 50% definition of improvement after etanercept treatment. To our surprise, 15 of 22 patients (68.2%) had a response with 70% definition of improvement. Moreover, in DAS28, eight patients were evaluated as having a good response and there were no patients with a poor response to etanercept. Treatment had to be stopped in one patient who developed joint contracture during the study period, but there were no significant adverse events in the other patients. In conclusion, treatment with etanercept leads to significant improvement in patients with active polyarticular JIA in Japan. Etanercept is well tolerated by pediatric patients as well as adults.