A 1-year-old Welch Pony filly presented to the Medicine Service at Cornell University's Large Animal Hospital with a 3-week history of intermittent fevers, lethargy, petechiae and shifting lameness. Prior antibiotic therapy failed to result in resolution of the clinical signs. Physical exam revealed left and right sided, grade III out of VI, apical, holosystolic heart murmurs. In addition, a grade IV out of VI "to and fro" murmur was auscultated over the left heart base. Clinical pathology identified a mild anemia, leukocytosis with a mature neutrophilia, and a severe thrombocytopenia. The presence of a fever of unknown origin, heart murmurs and leukocytosis led to a presumptive diagnosis of bacterial endocarditis. Other differential diagnoses considered included chronic inflammation, chronic infection, neoplasia, and immune mediated disease. Two-dimensional echocardiography identified severe vegetative lesions on the mitral, tricuspid, and aortic valves. Color flow Doppler showed mitral, tricuspid and aortic insufficiency, along with mild aortic stenosis. Due to the grave prognosis, the owners elected to have the pony euthanized. The diagnosis was confirmed with gross and histological post-mortem examination. This paper will discuss the pathogenesis of bacterial endocarditis and its potential consequences.