Isolated extramedullary relapse (EMR) after allogeneic hematopoietic stem cell trans-plantation (allo-HSCT) in acute myeloid leukemia (AML) is relatively rare and the clin-ical significance is still not well characterized, particularly in children. During or after chemotherapy, an uncommon event termed lineage switch, in which the lineage at onset of leukemia converts to another lineage at a later time, is observed in a subset of leukemia patients, but it is a rare event. We report a 10-year-old boy with isolated mediastinal EMR after allo-HSCT for AML who subsequently experienced BM relapse with the cell lineage switching from AML to T-cell acute lymphoblastic leukemia.