A 63-year-old man was analyzed for platypnea-orthodeoxia syndrome (POS). A complete obstruction due to bronchial carcinoma was found in the left main bronchus with bronchoscopy. After left sided pneumonectomy POS resolved completely. Historical reports suggest increased shunting through the left lung could occur in the upright position caused by decreased compression of the left pulmonary artery due to the central bronchial carcinoma. Partially absent hypoxic vasoconstriction was confirmed in this case and suggests (without a relevant shunt through a patent foramen ovale) this 'historical' hypothesis could explain the POS due to increased shunting in the upright position in this patient.