Treatment-free remission (TFR) has emerged as a new goal in the treatment of chronic myeloid leukemia (CML). TFR is considered a safe intervention because patients who experienced molecular relapse usually responded well to tyrosine kinase inhibitors resumption and regained molecular response quite efficiently. Nevertheless, there have been reports of occurrence of blast crisis during TFR.We report a case of sudden lymphoid blast crisis in a CML patient who had been in TFR for 21 months without any prior molecular loss. Whole-exon sequencing identified a frameshift mutation of SETD2. In addition, we reviewed the current literature on cases of blast crisis in TFR. Only eleven cases of blast crisis have been reported among thousands of patients who discontinued tyrosine kinase inhibitor (TKI) therapy, including our patient. Of these cases, nine presented with lymphoid blast crisis. Additional gene mutations are frequently observed.This case, along with others, emphasizes the necessity of implementing a long-term monitoring strategy following TKI discontinuation due to the potential for late onset of blast crisis. Systematic genetic studies in patients failing TFR should be properly carried out to further understand the mechanism and, eventually, to predict or prevent such adverse event in patients in TFR.© 2024 S. Karger AG, Basel.