We report a case of BOOP with severe hypoxemia and orthodeoxia. Chest X-ray showed bilateral infiltrative pneumonia and computed tomography scan revealed consolidations on lower lobes. Diagnosis of right-to-left shunt was made on the increase of alveolar-arterial oxygen difference under FIO2 100%, accented in standing position. Contrast-enhanced echocardiography and technetium-99 macroaggregated albumin body scanning did not show anatomical right-to-left shunt and permitted the diagnosis of intrapulmonary shunt at the capillary level because of intra-alveolar organisation. Treatment with corticosteroids and cyclophosphamide led to clinical, and radiological improvement with correction of the hypoxemia and the shunt.