"Cassette Player '03" Bilinski, a 7-month-old Thoroughbred colt, presented to the Cornell University Large Animal Medicine Service on August 25, 2003 with the chief complaint of a heart murmur. Three murmurs were heard upon cardiac auscultation: a grade III/VI systolic murmur over the aortic and mitral valve area, a grade III/VI systolic murmur over the pulmonic valve area, and a grade III/VI systolic murmur over the tricuspid valve area. Findings of a cardiology consult included a membranous ventricular septal defect with bi-directional shunting, mitral valve dysplasia, tricuspid valve dysplasia, and significant pulmonary hypertension. Due to the poor prognosis for athletic performance, the foal was euthanized and a necropsy was performed. This paper will discuss ventricular septal defects in the horse along with their clinical manifestations and varying prognoses. Factors which play a role in causing pulmonary hypertension and shunt reversal will also be discussed, particularly as they apply to this case.