A nine-year-old golden-headed lion tamarin (Leontopithecus chrysomelas) presented with non-specific signs of illness including pale mucous membranes, un-thriftiness, and a recent non-healing pressure sore related to chronic hindlimb neuropathy. Physical examination revealed marked peripheral lymphadenopathy. Bloodwork revealed a regenerative anemia (Hct=20.7%, RBC=3.38×106/µl, reticulocytes=11%), leukocytosis with a left shift (WBC=20.1×103/µl, 89% segmented neutrophils, 4% band neutrophils, 7% lymphocytes), and hyperglobulinemia (6.1 g/dl). Lymph node fine needle aspirates were indicative of suppurative inflammation, although no associated infectious or inflammatory foci were identified on physical examination or radiographs. Antibiotic therapy was initiated with sulfamethoxazole and trimethoprim (Sulfatrim pediatric suspension, 30 mg/kg PO BID; Alpharma USPD Inc., Baltimore, MD). Bloodwork seven days following initial presentation showed no improvement. A second antibiotic was added at this time (enrofloxacin, 10 mg/kg IM every 24 h; Baytril, Bayer Healthcare LLC, Shawnee Mission, KS). A recheck exam on day 16 showed no change in the lymphadenopathy and minimal improvement in blood parameters. An ultrasound exam on day 19 revealed a 3×1.8 cm mass caudal to the right kidney with homogeneous echogenicity. An ultrasound-guided aspirate of the mass revealed a pleomorphic population of reactive inflammatory cells and was considered consistent with an abscess. An exploratory laparotomy was performed two days later. A 2×2×2.5 cm, brown, encapsulated, fluctuant mass was found adhered to the vena cava and right kidney. The mass was excised and had no obvious organ of origin. Histopathology revealed the mass consisted of channels containing red blood cells, hematopoietic cell clusters, and occasional multinucleate cells with nodules of lymphoid tissue, which was considered to be consistent with myeloproliferative disease. Surgical recovery was uncomplicated. Bloodwork 13 days following surgery showed slight improvement of the anemia and leukocytosis; however, the animal continued to appear depressed and lethargic. The tamarin was humanely euthanatized 16 days following surgery due to the poor prognosis and a decline in the animal’s attitude. Serology for HSV-1, SRV, STLV, Herpes tamarinus, H. saimiri, squirrel monkey CMV, LCM IgG, and Epstein-Barr were all negative at the time of euthanasia. Necropsy supported the biopsy diagnosis with histopathologic evidence of disseminated myeloid hyperplasia affecting the spleen, lymph nodes, and bone marrow. In contrast to neoplasia of lymphoid origin, myeloproliferative disorders have been rarely reported in the literature in non-human primates. To the authors’ knowledge this is the first report in the literature of a case affecting a golden-headed lion tamarin. The case is of particular interest due to the difficulty in establishing a definitive diagnosis, particularly as aspirates and bloodwork suggested an infectious etiology. Myeloproliferative disease should be considered as a differential diagnosis in any case of lymphadenopathy of unknown etiology.