Hematological failure following treatment with Chimeric AntigenReceptor T-cell (CAR-T), now denominated Immune Effector Cell-Associated Hematotoxicity (ICAHT), is emerging as one of the most challenging toxicities. The incidence of a prolonged ICAHT may rise in 16% to 53% of patients, and its management has not been completely standardized despite some efforts in this regard. 1 Treatments may include granulocyte colony-stimulating factor (G-CSF), thrombopoietin receptor agonists (TPO-RAs), steroids, or careful monitoring. Among possible therapy approaches, there is increasing reporting of hematopoietic stem cell boosts (HSCB) to overcome this severe condition. 2 According to current EHA/EBMT recommendations, an autologous infusion of HSCB with no prior conditioning may be indicated in cases of persistent severe ICAHT that are refractory to G-CSF and last beyond day 14. Alternatively, if no autologous stem cells are available and if a grade 4 ICAHT persists beyond day 30, an allogeneic hematopoietic stem cell transplantation (HSCT) may be considered as a last chance in tailored conditions. 1 Here, we describe the case of a patient who underwent allogeneic HSCB for persistent and severe ICAHT. Subsequently, the patient recovered from cytopenias but also experienced a relapse of graft versus host disease (GVHD).A 43-year-old man presented at the Hematology Emergency Department with severe trilinear cytopenia, sepsis caused by Escherichia coli, and soft tissue infection in his left leg. The patient was experiencing severe and prolonged ICAHT.Three years before that date, the patient was diagnosed with a mantle cell lymphoma (MCL), stage IV, which was treated with several lines of chemoimmunotherapy, including autologous HSCT, ibrutinib, and radiotherapy, resulting in good partial remission. The outcome was then consolidated with allogeneic HSCT from his HLA-identical sister: the source of stem cells was peripheral blood, the conditioning consisted of a reduced intensity regimen, and prophylaxis against GVHD was a combination of cyclophosphamide, cyclosporine, and mycophenolate mofetil. The engraftment was fully achieved for neutrophils, platelets, and reticulocytes at day +26, +35, and +40,