ObjectiveDiffuse alveolar hemorrhage (DAH) is an uncommon complication of primary antiphospholipid syndrome (APS). We aimed to describe the clinical characteristics, treatment, and outcomes of primary APS–associated DAH in a single center.MethodsWe conducted a retrospective review of all adults with primary APS–associated DAH evaluated at Mayo Clinic over a 15‐year period. DAH was defined as bilateral pulmonary infiltrates and bronchoalveolar lavage (BAL) fluid documenting progressively bloody returns and/or the presence of >20% hemosiderin‐laden macrophages. Patients with other causes of DAH were excluded.ResultsEighteen patients were identified (median age 43 years). Capillaritis was present in surgical lung biopsy samples of 3 patients. BAL differential cell counts revealed predominantly neutrophils. All patients were treated initially with glucocorticoids. Cyclophosphamide (CYC) was used in 8 patients; complete remission was achieved in 3 patients treated with CYC alone and in 1 patient receiving combination therapy with rituximab (RTX). RTX was used in 9 patients; 2 patients achieved remission with RTX alone, whereas 3 patients required combination therapy with CYC or mycophenolate mofetil (MMF). No patient achieved complete remission while receiving single therapy with MMF, azathioprine, or plasma exchange. Intravenous gamma globulin therapy was administered in 5 patients; no patient achieved control of the disease. Six patients died, all because of complications related to uncontrolled DAH or its therapy.ConclusionWe present the largest case series of primary APS–associated DAH reported in the literature. DAH carries a very poor prognosis and therapeutic options are limited. Immunosuppression with either CYC or RTX is associated with the highest likelihood of remission induction and should be considered early.