Purpose: Klinefelter's syndrome, the most common congenital abnormality causing primary hypogonadism, is associated with breast cancer, germcell tumours, and nonhodgkin lymphomas. A 53-year-old male came with fatigue, poor appetite, weakness, 51 pounds of weight loss, and night sweats in the last year. He started having nausea, vomiting with specks of blood and sharp abdominal pain mainly in epigastric region. He denied any medication use. He used to smoke. Family history was noncontributory. On admission, his vitals were stable. Epigastric tenderness was present, otherwise his examination was benign. His hemoglobin was 7.3 with MCV of 73.6, albumin was 1.9 and INR was 1.43. HIV test was negative. LDH was 152, hep B and hep C tests were negative. CT chest, abdomen and pelvis with contrast revealed splenomegaly, mesenteric and retroperitoneal lymphadenopathy. Endoscopy showed deep ulcer on the lesser curvature of the stomach approximately 7 cm from the GE junction with irregular friable borders and volcano appearance, and acute gastritis was found in the antrum. Biopsy showed diffuse large B cell lymphoma. Bone marrow biopsy was negative for lymphoma. Cytogenetic analysis on bone marrow showed XXY (Klinefelter syndrome). PETCT revealed diffuse irregular thickening with intense uptake in the proximal stomach consistent with gastric lymphoma. Multiple conglomerate lymph nodes with intense uptake in the gastrohepatic ligament, gastrosplenic space, preaortic, aortocaval, peripancreatic, celiac, mesenteric and left common iliac regions, the largest measuring about 6 cm, in the left periaortic region and abutting the pancreatic body and tail. All were consistent with lymphoma. Mild splenomegaly was present without abnormal uptake. Stage IIIB DLBCL (diffuse large B cell lymphoma) was diagnosed and the patient was started on treatment with 6 cycles of R-CHOP. Repeat PETCT showed incredible response of the gastric and nodal lymphomatous lesions to chemotherapy compared to the previous one. Repeat endoscopy showed complete resolution of previous lesions. Klinefelter's syndrome patients may be at substantially elevated risks for non-Hodgkin's lymphoma. Studies have shown lymphocytes in Klinefelter's syndrome patients have higher sensitivity than normal control cells when exposed to radiation or mitomycin C. Cells of Klinefelter's syndrome were 3-10 times more easily transformable than normal cells. Review of the literature suggested men with Klinefelter's syndrome have increased incidence of non-Hodgkin's lymphomas. They suggested a possible combination of immunological deficency, endocrinological factors and inability to control hematopoiesis as an etiological factor.