Introduction: The advent of adoptive cellular therapy utilizing autologous T cells expressing chimeric antigen receptors (CAR-T) against CD19 has revolutionized relapsed and refractory B cell lymphoma treatment. CAR-T cell therapy, however, is not without toxicity. Other than cytokine release syndrome and neurotoxicity, hematologic toxicity occurs commonly as well. Grade 3 and above prolonged neutropenia and thrombocytopenia (longer than 4 weeks since T cell infusion) was seen in 15 to 40% and 18 to 40% of patients treated with CD19 CAR T cells in clinical trials respectively. The exact mechanism underlying the pathogenesis of prolonged hematologic toxicity in CAR-T cell therapy is not well understood. Interestingly, we identified four cases of prolonged cytopenias after CAR-T cell therapy due to the presence of myelodysplastic syndrome in patients receiving this treatment for relapsed refractory B cell lymphoma. Methods: We performed a retrospective review of four patients with relapsed refractory aggressive B cell lymphoma treated at our institution with CD19 directed autologous CAR-T cells. Results: Median age was 74 (range: 57-76). Two patients had GCB type, 2 had non-GCB type DLBCL. The median number of lines of therapy prior to CAR-T cell therapy was 5. All four patients had prior autologous stem cell transplantation. Median time to MDS diagnosis from CAR T infusion was 3 months (range: 2 to 3). One patient developed MDS after additional chemotherapy was given for relapsed disease. One patient had evidence of a dysplastic clone prior to CAR-T cell therapy. The remaining two patients did not have evidence of MDS until immediately after CAR-T cell therapy. Two patients developed severe CRS requiring tocilizumab and corticosteroids. One patient showed partial response to CAR-T cell therapy, while three others achieved CR, two of which have since relapsed. Discussion: We identified four patients with prolonged hematologic toxicity after CAR-T cell therapy due to MDS. These patients have multiple risk factors for the development of MDS including advanced age, and heavy pretreatment. In fact, one patient had evidence of MDS prior to CAR-T cell therapy, underscoring the potential benefit of providing CAR-T cell therapy earlier in the course of treatment. In addition, our findings suggest that a bone marrow study should be performed in patients with prolonged cytopenias post CAR T cell therapy. It is not clear if there is any direct contribution of CAR T cells and the resulting immune system disturbance to the development of MDS. Long term follow up of all patients receiving CAR T cell therapy is therefore crucial to assessment true incidence of MDS and establish association. Keywords: B-cell lymphoma; CD19. Disclosures: Herrera, A: Consultant Advisory Role: Gilead. Siddiqi, T: Consultant Advisory Role: Juno Therapeutics. Budde, E: Consultant Advisory Role: Gilead. 480 – WITHDRAWN