BACKGROUND Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening syndrome of systemic hyper-inflammation, caused by an excessive cytokine secretion, secondary to an uncontrolled proliferation of lymphocytes and macrophages, and leading to vascular endothelial injury and multi-organ failure. HLH is either primary/familial due to genetic mutations in the genes coding for the CD8+ and NK T cells cytotoxic proteins or is secondary to infection, malignancy, or autoimmune disorders. Timely diagnosis using the HLH-2004 criteria and prompt initiation of treatment for HLH is essential for the survival of affected patients. Adults with HLH have poor outcomes even with aggressive treatment. CASE REPORT Our patient was a 48-year-old man who presented with altered mental status. He was tachycardic and tachypneic, and quickly developed acute hypoxemic respiratory failure requiring mechanical ventilation. Computed tomography (CT) of the chest and abdomen showed bilateral pleural effusion, ascites, and heterogeneous splenomegaly. Laboratory workup revealed anemia, thrombocytopenia, severe hyperferritinemia, hypofibrinogenemia, and hypertriglyceridemia. Pleural fluid analysis showed a lymphocytic exudate, with T cell predominance on flow cytometry. A T cell rearrangement study of the pleural fluid was positive. Bone marrow biopsy showed histiocytes with hemophagocytic activity. The diagnosis of HLH secondary to T cell lymphoma was made, and the patient was treated with dexamethasone and etoposide. A few hours later, the patient had a cardiac arrest, and laboratory findings suggestive of tumor lysis syndrome (TLS) were discovered. The patient died of refractory shock one day later, and the cytomegalovirus (CMV) PCR result was positive during that day. CONCLUSIONS Adults with HLH have poor outcomes even with aggressive treatment. Additional focus on the management of HLH should shift towards preventing complications such as TLS. More studies should focus on post-treatment outcomes of HLH secondary to malignancy to improve the management and prognosis.