This study describes a case of tetralogy of Fallot in a lamb showing failure to thrive and signs of respiratory distress. Physical examination, electrocardiography, thoracic radiographies, echocardiography and cardiac troponin I evaluation were performed. The value of cardiac troponin I was compared with the values of 10 healthy lambs of the same age and breed, and the affected animal demonstrated an increase in cardiac troponin I. Due to the poor prognosis, euthanasia was indicated, and necropsy confirmed the diagnosis. This is the first report of an increase in cardiac troponin I in a lamb with tetralogy of Fallot.