Pulmonary manifestations of the antiphospholipid syndrome (APS) include pulmonary hypertension, pulmonary embolism, and in patients with the catastrophic APS, respiratory insufficiency associated with diffuse pulmonary infiltrates. There are few descriptions of pathological findings associated with these pulmonary complications. We document pulmonary capillaritis, recurrent microvascular thrombosis and alveolar hemorrhage in patients with APS. These patients have a wide spectrum of clinical presentations, ranging from recurrent episodes of fever, minimal hemoptysis, and mild dyspnea, to respiratory insufficiency requiring mechanical ventilation. Pulmonary involvement can occur alone or simultaneously with other manifestations of APS and may range from mild to severe. Pulmonary capillaritis and alveolar hemorrhage are complications of APS, and whether they are causally related to recurrent thromboembolism and pulmonary hypertension remains undetermined.