This patient presented with clinical and radiographic features of diffuse alveolar hemorrhage, an entity with a broad differential diagnosis and a potentially catastrophic outcome, with an acute mortality of up to 50%. Prompt diagnosis should therefore be pursued concurrently with stabilization of the patient. In this patient, drug use (ie, crack cocaine inhalation, propylthiouracil, diphenylhydantoin, penicillamine, amiodarone, nitrofurantoin) and environmental exposures (ie, hydrocarbons, infection) were excluded. Given the paucity of systemic findings, the differential diagnosis included anti-glomerular basement membrane disease (Goodpasture syndrome) confined to the lung, isolated pulmonary capillaritis with or without autoantibodies, antineutrophil cytoplasmic antibody-associated vasculitides (Wegener granulomatosis with isolated pulmonary involvement, or microscopic polyangiitis), Henoch-Schönlein purpura, and cryoglobulinemia. Less likely syndromes in this case included systemic lupus erythematosus, collagen vascular diseases such as systemic sclerosis, dermatomyositis, and antiphospholipid syndrome. Sending serum for ANCA, antinuclear antibodies, and anti-GBM antibodies, as well as obtaining a careful history and routine electrolyte, renal function, hematology and coagulation studies, were therefore critical in narrowing the differential diagnosis.