Inverted left atrial appendage (ILAA) is a rare manifestation. The case is reported of a patient with a history of factor V Leiden who underwent a Ross operation. An intra-atrial mass was discovered one month postoperatively and suspected to be a thrombus. Despite six months' anticoagulant treatment, the intra-atrial mass persisted. Ultimately, the patient was reoperated on due to hemostatic risks factors, and an ILAA was found. The diagnosis of ILAA remains a major challenge. Despite widespread use of postoperative echocardiography, the few reported cases confirm ILAA to be a rare phenomenon. The finding at post-cardiotomy echocardiography of a left atrial mass associated with an absence of the left appendage is highly suggestive of a diagnosis of ILAA. As the incidence of complications remains to be defined, treatment remains controversial, but embolic risk factors such as hemostatic disease or atrial fibrillation invariably lead to surgery. In an asymptomatic patient without thromboembolic risks factors, a conservative approach should be considered, because of the risk of reoperation.