"Spencer", an 11 year old Thoroughbred gelding, presented to the Equine Hospital at Cornell University on January 28, 2003 for evaluation of chronic weight loss of 6 months duration. "Spencer" was moved from Colorado to Connecticut in August of 2002, at which time he began to lose weight despite excellent care by his owner. The horse was referred for evaluation of the chronic weight loss, depression, persistent polycythemia, and an unexplained episode of rhabdomyolysis (CK=14,000). This paper will focus on the differential diagnosis for weight loss in a horse, how each was considered, and sequential diagnostic testing to arrive at the presumptive antemortem diagnosis of diffuse alimentary lymphoma, and the complicating factors including difficulty in distinguishing alimentary lymphoma from diffuse inflammatory bowel disease. Although not definitively proven, a histopathologic diagnosis of inclusion body myositis, and clinical diagnosis of polycythemia were also made in this case.