Background: There is no standard treatment for acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) relapsing after allogeneic stem cell transplant (SCT). Options include combination chemotherapy, withdrawal of immunosuppression, donor lymphocyte infusion (DLI), and second SCT. Previous studies have used fludarabine, cytarabine, and granulocyte colony stimulating factor (FLAG) or second SCT separately for salvage therapy. Our institution uses FLAG followed by SCT from the same donor (FLAG/SCT) in this setting. We report a retrospective study of FLAG/SCT in MDS and AML patients relapsed after SCT. Methods: Patients who received FLAG/SCT for treatment of relapsed AML or MDS between 2009 and 2018 were identified using the bone marrow transplant database at University of California Davis. Their baseline characteristics and outcomes were determined using the electronic medical record. Descriptive statistics and Kaplan-Meier survival analysis were used to describe patients, rates of graft-versus-host disease (GvHD) and estimate survival times. Results: Nineteen patients received FLAG/SCT for AML (n=18) and MDS (n=1). Median time to relapse from first SCT was 145 days (range 41 to 960 days). Prior to FLAG/SCT, 17 patients had medullary relapse (median bone marrow blasts 27%; range 7-85%). Two patients had extramedullary relapse. Eighteen (94.7%) patients achieved complete remission (CR) after FLAG/SCT. Median follow-up time was 354 days from the first day of FLAG/SCT (range 7 to 2144 days). Six patients (31.6%) relapsed with median time to relapse of 334 days (range 78 to 679 days) after treatment. Overall survival at 2 years was 52.5%. Causes of death were relapsed AML (n=4; 21.1%), infection (n=4; 21.1%) complications of GvHD (n=3,15.8 %), and brain herniation (n=1, 5.3%). Acute GvHD grade I-IV and new onset chronic GvHD occurred in thirteen (68.4%) and eight patients (42.1%), respectively. Conclusions: FLAG/SCT for AML and MDS relapsing after SCT resulted in a high remission rate. The overall survival of over two years suggests that the second SCT augmented the graft versus leukemia effect. The GvHD rate increased after second SCT, but the rate and severity were manageable. FLAG/ SCT is a reasonable option for relapsed AML and MDS after SCT.