A 3-year-old girl presenting with a huge mediastinal tumor and massive pleural effusion, was diagnosed with stage III γδ T-cell lymphoblastic lymphoma (γδ T-LBL) by flow cytometry of effusion cells. Four courses of chemotherapy achieved complete remission, and 5/6 HLA allele-matched cord blood transplantation was performed with success. The patient remains in first complete remission >16 months after transplantation. γδ T-LBL is very rare, especially that of mediastinal origin, and is acknowledged as having an extremely poor clinical outcome. The present case study is the first to report the successful treatment of mediastinal γδ T-LBL in a toddler.