CASE PRESENTATION: An 80-year old male with chronic rheumatoid arthritis treated with prednisone and hydroxychloroquine was hospitalized due to worsening dyspnea and 20 pound- weight loss in 2 months. CT revealed multi-focal ground glass infiltrates, mediastinal adenopathy, and splenomegaly (20cm.). His bronchoscopy was unrevealing and was treated for community-acquired pneumonia. He had pancytopenia with WBC count 1.8 K/mcl and platelets 47 K/mcl. During outpatient bone marrow biopsy 4 weeks later, he was found to be hypotensive, forgetful, anorexic and diaphoretic. He was transferred to the intensive care unit. Physical examination showed hepatomegaly, splenomegaly, axillary lymphadenopathy and hypotension. Since clinical suspicion for MAS was high, appropriate laboratory tests were ordered and he was started on dexamethasone 10 mg every 8 hours. Appropriate blood products were transfused. He deteriorated rapidly requiring several vasopressors and family decided to withdraw care on day 5. Our patient met 5/8 diagnostic criteria used in HLH-2004 trial: Splenomegaly, peripheral blood cytopenia (WBC - 1.3 K/mcl, Hemoglobin 7.0 gm/dL, and platelets 19 K/mcl), Hypofibrinogenemia - 92 mg/dL, Macrophages containing cellular material in the bone marrow and Ferritin level > 7500 ng/ mL. We believe his initial presentation was related to MAS and our case highlights the difficulty in diagnosing this syndrome.