The aim of this study was to assess the outcome of treatment with 2-chlorodeoxyadenosine (2-CdA) in pediatric patients with Langerhans cell histiocytosis (LCH) in Japan. We retrospectively identified 17 pediatric LCH patients treated with 2-CdA. All patients were refractory or reactivated cases who had been initially treated according to the JLSG-02 protocol of the Japan LCH study group. At initiation of 2-CdA therapy, 4 patients had primary refractory multisystem (MS) disease with risk organ (RO) involvement (MS+), 9 patients had reactivated MS disease [5 MS+ and 4 without RO involvement (MS-)], and the remaining 4 patients had refractory/reactivated multifocal bone disease (MFB). Treatment with 2-CdA (4-9 mg/m(2)/day) was administered on 2-5 consecutive days and repeated every 3-4 weeks for a period that ranged from 2 to 12 months. Four primary refractory patients were treated with 2-CdA combined with high dose of cytarabine. In MS+ patients, response to treatment was observed in 5 of the 9 patients. In MS-/MFB patients, 5 of the 8 patients showed response to treatment. In the patients who were primary refractory or had reactivation during initial chemotherapy, 4 of 10 patients showed good response. On the other hand, in the patients having reactivation while off therapy, 6 of 7 patients showed good response. These findings suggest that 2-CdA is effective for reactivated LCH while off therapy.