Background: LGL - often called LGL leukemia - is a clonal disorder of T or NK cells often associated with cytopenias, autoimmunity, splenomegaly, and B symptoms. There are a limited number of studies of benign LGL expansion after alloBMT, some suggesting an association with improved transplant-related outcomes. In contrast, clinically significant LGL leukemia after alloBMT is only described in case reports. Methods: We cross referenced all patients receiving an alloBMT at Johns Hopkins since 2010 with patients who were evaluated for LGL expansion by peripheral blood (PB) flow cytometry (FC) since 2012. Results: There were 1930 alloBMTs from 1/1/10 to 7/1/19. PB FC for suspected LGL was sent on 153 unique patients after alloBMT, usually in the setting of cytopenias (97%). Median age was 59. 69 (45%) had LGL expansion (LGL+) at a median 194 days after alloBMT. Among LGL+, 53 (77%) had an absolute neutrophil count (ANC) < 1500. The majority of the alloBMTs were non-myeloablative (NMA) (97%), related (88%), and haploidentical (89%), consistent with our center’s characteristics. Graft vs host disease (GVHD) prophylaxis was post-transplant cyclophosphamide (PTCy), mycophenolate mofetil, and tacrolimus or sirolimus. 64 (93%) cases were T cell LGL (T-LGL) and 5 were NK cell. Of those with T-LGL, 43 were assessed for T cell receptor clonality. 21% were clonal, 53% oligoclonal, 5% polyclonal, and 21% indeterminate. There were no significant demographic or transplant-related differences between LGL+ and LGL- in our 153 patient cohort. LGL+ were more likely to have had CMV viremia (75% vs 26%, p < 0.0001), but not acute or chronic GVHD. LGL+ had higher lymphocyte counts (1520/cu mm vs 495, p < 0.0001) and a trend toward more neutropenia (77% vs 63%, p = 0.07). There were no differences in overall survival, relapse, or non-relapse mortality. 29 (42%) of LGL+ received immunosuppressive therapy (IST) for cytopenias. First line treatment was corticosteroids for 24 (83%). 65% had normalization of ANC with first line treatment, compared to improvements in anemia in 15% and thrombocytopenia in 36%. 34% of those treated required ≥2 lines of treatment. Conclusions: In contrast to prior studies, where LGL after alloBMT was asymptomatic and associated with improved transplant outcomes, we identified a high rate of LGL with cytopenias and no improvement in transplant outcomes. Neutropenia was common in LGL+ and usually improved with IST.