To investigate the relationship between CD4+CD25-CD69+ T cells and leukemia relapse, we first examined the frequency of these cells in the bone marrow from 29 patients who were treated for a malignant hematological disease with allo-HSCT, including patients undergoing hematological relapse (n = 22) and those with a positive MRD status (n = 7). Here, CD69 is linked to hematologic disorder.