ever of unknown origin is caused by infection in 43% of cases, neoplasia in 22%, immune-mediated diseases in 6.5%, and unknown etiology in 9.5%1. A 14 year old Paint gelding presented to Cornell’s Equine Hospital with a six week history recurrent fever of unknown origin and a 3 day history of variable pelvic limb lameness. Thorough diagnostic testing often identifies the etiology of the patient’s clinical signs. Blood panels, multiple imaging modalities, and cultures form the basis of the diagnostic approach. Primary differentials in the patient included bastard strangles, purpura hemorrhagica, Anaplasma phagocytophilum, lymphoma, and a tendon sheath infection. Despite extensive diagnostic testing, no definitive etiology for the patient’s signs could be established, and he was euthanized. In these cases, a necropsy should be performed to reconcile the patient’s clinical signs and test results to help edify the clinician and improve the efficiency of diagnosis in the future.