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Background: Hemophagocytic lymphohistiocytosis is a rare rapidly progressive condition of immune dysregulation characterized by a hyperinflammatory response and uncontrolled T-cell and macrophage activation. Concomitant involvement of acute myeloid leukemia (AML) and hemophagocytic lymphohistiocytosis (HLH) is even more rare and presents a challenge for clinical management. Clinical Course: We present a 76-year-old man with fatigue, shortness of breath, thrombocytopenia and leukocytosis. CT revealed splenomegaly, enlarged mediastinal lymph nodes, and PET study showed bone marrow hypermetabolic activity. Hematology work up established diagnosis of AML, concomitant HLH and tumor lysis syndrome. Considering the primary problem is AML, CHEMOTHERAPY with Cytarabine was initiated. Due to continuous increasing bilirubin, liver biopsy was performed. Later the patient developed septicemia and expired. Pathology Findings: Microscopically, it revealed extensive cholestasis, bile ductule proliferation, and ballooning degeneration. The iron stain demonstrated 4+ iron and hemosiderin depositions. Within hepatic sinusoids, there were increased immature monocytic cells in single cells or in clusters, with slightly open chromatin, irregular nuclear contours, and relatively abundant cytoplasm, which were positive for CD14, CD68, and negative for CD34, CD56, and MPO. CD3 highlighted T-cells in portal tracts and occasionally in sinusoids. In hepatic sinusoids, hemophagocytic lymphohistiocytosis was seen, with macrophages or monocytes engulfing red cells. The findings are consistent with acute myeloid leukemia with monocytic differentiation, involving hepatic sinusoids, with concurrent hemophagocytic lymphohistiocytosis. Molecular studies including JAK2V617F mutation analysis, fluorescence in situ hybridization for BCR/ABL1, and cytogenetic analysis were unremarkable. Discussion: Hemophagocytic lymphohistiocytosis can be found in up to 10% of AML patients going through chemotherapy and frequently leads to rapid mortality. Since clinical treatment for leukemia and HLH is drastically different, recognizing the morphologic changes and the immunophenotypic features of the infiltrates are critical for a rapid diagnosis and a prompt institution of treatment. [ABSTRACT FROM AUTHOR]