Lymphocytes are an important component of the equine immune system. Like all cells within the body, lymphocytes have the potential to undergo neoplastic (cancerous) transformation that results in uncontrolled regulation and growth.

Lymphosarcoma is the proliferation of neoplastic lymphocytes. Equine lymphosarcoma is relatively common, but the exact incidence is unknown. The majority of cases occur in horses aged 4 to 15 years, but cases have been described in horses of all ages.

Epidemiologic studies suggest no apparent gender or breed predisposition. Like many equine cancers, the cause of lymphosarcoma is rarely identified, but certain bacteria and viruses have been implicated in its development.

Four anatomical categories are frequently utilized for classification of lymphosarcoma: multicentric (generalized or within multiple locations), thymic (mediastinal, within the chest cavity), alimentary (intestinal), or cutaneous (skin or extranodal)