A 5-month-old male cynomolgus monkey (Macaca fascicularis) with facial deformation and proptosis of the left eye was euthanized due to a rapidly-growing peribulbar and retro-orbital mass. The mass invaded the soft tissue behind the right eye and extended ventro-medially through the lacrimal, ethmoid, maxillary and frontal bones. Caudo-dorsal extention of the mass through the left frontal bone and into the meninges and parenchyma of the frontal cerebral lobe was observed. Histologically, the cell population making up the mass was variable, but most of the neoplastic cells were blast-like with a large central round to ovoid nucleus, active coarsely stippled euchromatic chromatin, 1-4 prominent nucleoli and sparse cytoplasm. Interspersed between these neoplastic lymphocytes were numerous aberrant mitoses and histiocytes with foamy cytoplasm producing a "starry-sky" pattern. Cells further away from the central neoplastic core and closer to fragmented bone surfaces had plasmacytoid features with an eccentric hyperchromatic nucleus and abundant eosinophilic cytoplasm. Electron microscopy confirmed the formentioned cell types. Morphometric analysis of tumor cell indicated similarities to cells of human Burkitt's lymphoma. The tumor cells indicated similarities to cells of human Burkitt's lymphoma. The tumor was diagnosed as a malignant lymphoma of unknown etiology, similar to those seen in African children with Burkitt's lymphoma.