Antiphospholipid syndrome (APS) is an autoimmune disease characterized by the presence of circulating autoantibodies against phospholipid-binding plasma proteins, leading to an increased risk of thrombosis and pregnancy loss. The most common manifestation of lung disease in APS is pulmonary embolism, which may often be the presenting symptom. We present a 30-year-old man with probable primary APS (with no history of thromboses) presenting with diffuse alveolar hemorrhage, an uncommon presentation. He was also found to have severe mitral valve regurgitation and during valve replacement surgery had cardiac vegetations compatible with a presentation of Libman-Sacks endocarditis. There are only 21 other reported cases of diffuse alveolar hemorrhage occurring as a result of APS. This is the first case of Libman-Sacks endocarditis in the setting of probable APS and alveolar hemorrhage.Diffuse alveolar hemorrhage should be considered as a nonthrombotic manifestation of APS, even in the absence of known thromboses, and may be the presenting symptom.