Rituximab (Rituxan, Mabthera) is an anti-CD20 monoclonal antibody that mediates killing of CD20+ B-lymphocytes by a combination of complement-mediated lysis and antibody-dependent cytotoxicity [1]. It is considered a safe medication, and infusion-related side effects are generally manageable. These include fever, chills and rigors, and they occur in 50% of patients [2]. Rituximab has been considered a nontoxic alternative to chemotherapy regimens and has become part of standard therapy for patients with CD20-expressing B-cell lymphoma [3]. In patients with a large number of CD20+ cells, tumor lysis may occur. Acute tumor lysis syndrome (TLS) caused by rituximab has been reported in two patients with chronic lymphocytic leukemia [4,5] and in three patients with nonHodgkin's lymphoma (NHL) [4,6,7]. We report another case of a patient with Burkitt's lymphoma who experienced acute TLS upon treatment with rituximab.A 53-year-old man presented to a hospital in Saudi Arabia in January 2003 with melena and abdominal pain. He reported abdominal pain and easy fatiguability of 10-day duration with no other symptoms. Esophagogastroduodenoscopy and biopsy of a gastric ulcer showed gastric lymphoma, large B-cell type. Complete blood count (CBC) showed severe anemia and 19% immature cells, and bone marrow aspirate (BMA) showed infiltration with blasts. Computerized tomography (CT) of the abdomen and pelvis revealed multiple enlarged intra-abdominal lymph nodes at the lesser curvature of the stomach and the mesenteric area. The patient was transfused with four units of packed red blood cells and was transferred to the American University of Beirut-Medical Center (AUB-MC).Upon presentation to AUB-MC, the patient was icteric and sick looking with pedal edema but no peripheral adenopathy or hepatosplenomegaly. His vital signs were within normal limits. CBC showed a white blood cell (WBC) count of 40.0 9 10 3 /lL with 64% blasts, hemoglobin of 10.8 g/ dL, and platelets 40 9 10 3 /lL. His chemistries showed the following: uric acid 14.7 mg/dL, creatinine 1 mg/dL, BUN 13 mg/dL, alkaline phosphatase 636 IU/L, c-GT 1,077 IU/L, SGPT 338 IU/L, LDH 7,266 IU/L (normal \ 480), INR 1, and fibrinogen 5.2 g/L. His echocardiogram was normal. CT scan showed an antral gastric mass and multiple abdominal lymph nodes and splenomegaly. Repeat BMA showed heavy infiltration by lymphoma cells. Flow cytometry on the peripheral blood showed B cell lymphoma with lambda light chain restriction. Bone marrow karyotype revealed t(8;22) (q24.3;q11.3)(9/11 cells) and der(1) in 82% of cells analyzed. The patient was diagnosed with Burkitt's lymphoma.Our patient was 164 cm in height and 106 kg in weight. He was started on allopurinol 300 mg daily, IV hydration and prednisone 100 mg PO. Repeat blood studies 24 h from admission showed a WBC count of 34.7 9 10 3 /uL with 81% blasts, uric acid 10.9 mg/dL, creatinine 1.2 mg/dL, BUN 25 mg/dL, Na 141 mmol/L, K 3.0 mmol/L, Cl 95 mmol/L, HCO3 25 mmol/L, Mg 2.03 mg/dL, Ca 8.5 mg/dL, PO4 3.8 mg/dL, and LDH 7,500 IU/L. The pa...