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Hepatic sinusoidal obstruction syndrome (SOS) has been reported in patients receiving high dose intravenous (IV) cyclophosphamide especially as a part of preparative regimen for hematopoietic stem cell transplantation. Hepatic SOS from low dose oral cyclophosphamide has not been reported. Here we present a rare case of hepatic SOS induced by low dose oral cyclophosphamide. A 78-year-old male with history of coronary artery disease status post coronary artery bypass graft (CABG) was initially seen in hematology clinic for evaluation of normocytic anemia. During workup, bone marrow biopsy revealed NK cell large granular lymphocytic leukemia (LGL). He was then started on low dose oral cyclophosphamide 50 mg daily. After 2 months, his dose was reduced to 25 mg daily due to severe neutropenia. He then tolerated cyclophosphamide well and his hemoglobin (Hb) improved from his baseline 8 gm/dl to 12 gm/dl in 4 months. After 6 months of treatment, he was admitted to the hospital for abdominal distension, ascites and 15 pounds weight gain over 3 weeks. Labs most prominent on admission were Hb of 12.5 gm/dL , WBC 5.3 x 103/µL, Platelets 197 x 103/µL, total bilirubin 1.6 mg/dL, aspartate aminotransferase (AST) 89 U/L , alanine aminotransferase (ALT) 35 U/L and alkaline phosphatase (ALP) 283 U/L. Workup of ascites included abdominal paracentesis which revealed serum ascites albumin gradient(SAAG) High dose of IV cyclophosphamide given as myeloablative therapy in combination of total body irradiation or busulfan (or other agents) in preparation for hematopoietic stem cell transplantation can induce hepatic SOS, which can be severe leading to acute liver failure and death. The diagnosis is usually based on clinical features of tender hepatomegaly, weight gain, ascites and jaundice. Liver biopsy is usually diagnostic. Mechanism of injury is related to direct toxic effects of cyclophosphamide on sinusoidal cells in the liver, causing their necrosis and release into the sinusoids, obstruction and obliteration of hepatic veins. This case demonstrates that hepatic SOS can also be caused by low dose oral cyclophosphamide. Given this rare occurrence, it is prudent for the clinicians to keep an open mind and consider hepatic SOS as a potential side effect even with oral cyclophosphamide. Disclosures Yacoub: Novartis: Speakers Bureau; Agios: Honoraria, Speakers Bureau; Incyte: Speakers Bureau; Hylapharm: Current equity holder in private company; Cara Therapeutics: Current equity holder in publicly-traded company; Ardelyx: Current equity holder in publicly-traded company; Dynavax: Current equity holder in publicly-traded company; Roche: Other: Support of parent study and funding of editorial support.