in solid organ malignancies, including melanoma, ovarian, breast, and especially, colorectal cancers, where a higher degree of tumor-infiltrating lymphocytes (TIL) predicts improved survival independent of TNM (tumor, node, metastases) staging [1]. TIL have also shown prognostic value in various lymphoid hematological malignancies. In AML, evidence of immune modulation is seen in the "graft-versus-leukemia" effects post-allogeneic SCT together with promising preclinical activity of novel immune therapies. Furthermore, higher peripheral blood lymphocyte recovery after induction chemotherapy [2] and allogeneic SCT for AML [3] have predicted improved overall survival (OS). The prognostic significance of immune status at diagnosis has not been defined in AML. We therefore investigated whether degree of T lymphocyte infiltration within CN-AML diagnostic bone marrow trephines, which we have labeled the "leukemia-infiltrating lymphocyte" (LIL) index, had prognostic value.Patients diagnosed with CN-AML at an adult tertiary center between 2006 and 2013, treated with intensive chemotherapy and who had suitable stored, diagnostic bone marrow trephines were analyzed with institutional ethics approval. Trephines were formalinfixed, decalcified in EDTA, and processed through to paraffin. Immunostaining for pan-T cell marker, CD3, and cytotoxic markers, CD8 and Granzyme B (GB), was performed at the time of analysis using routine methods. CD4 immunostaining was attempted, however high levels of non-specific staining precluded quantitative analysis. Positive cells were quantified using FijiV C image analysis software (v1.48o) from photographs taken at 3200 magnification of three representative areas of each trephine, expressed as a percentage of total cells and averaged over the three images.The cohort included 53 patients (51% male, median age 51 years (range 19-78)) followed-up over a median of 25.9 months (range 0.17-102.6 months). Most patients (96%) had de novo AML, 47 patients (89%) achieved CR (29 (62%) of whom relapsed), 21 (40%) underwent allogeneic SCT (10 in second CR) and 20 (38%) died. From a mean of 13,768 cells per trephine analyzed, median CD3% was 4.64% (range 0.56-37.24%) (Fig. 1A,B), CD8% was 5.08% (range 0.43-43.5%) and GB% was 0.34% (range 0-10.11%). In univariate Cox regression analyses, significant risk factors for death were higher aspirate blast% (P 5 0.006), primary refractory disease (P 5 0.03), and relapse (P 5 0.04). Increasing age (P 5 0.093), FLT3-ITD positive(pos) (P 5 0.099), and CD3% (P 5 0.079) approached significance. Gender, preceding myelodysplastic syndrome, allograft, NPM1 mutation (NPM1 mut ), CD8%, and GB% were not significant. In Cox regression multivariate analyses, combining T cell numbers with the relevant univariate factors (i.e., P < 0.1), higher CD3 and CD8 were independent predictors of OS (CD3: hazard ratio (HR) 0.929 for death, 95% CI 0.870-0.992, P 5 0.029; CD8: HR 0.920, 95% CI 0.869-0.973, P 5 0.004). Patients were divided into quartiles to determine whether incremental d...