Primary antiphospholipid syndrome (APS) is a thrombophilic disorder not associated with connective tissue diseases or with autoimmune diseases. We report a 39-year-old female with a mechanical prosthetic mitral valve and a primary antiphospholipid syndrome. Anticoagulation with warfarin was changed for tinzaparine. Four days after the change, the patients was admitted in cardiogenic shock. A transesophageal echocardiogram showed a mass in the prosthetic valve suggestive of prosthetic thrombosis. The patient required two valve replacements due to thrombosis. Her anticardiolipin antibody titers were 84 UPL. She was discharged with a coagulation international normalized ratio of three.