A 76-year-old man who had been diagnosed as having chronic bird-related hypersensitivity pneumonia had received prednisolone. His respiratory condition had been stable; however; O₂ saturation measured by pulse oximeter (SpO₂) was reduced to 91% (O₂ 1 L/min, nasal canula), but no cause could be detected on echocardiography or chest computed tomography imaging. In February 2013, he presented as an outpatient with a SpO₂ of 73% (O₂ 4 L/min, nasal canula, sitting position). We discovered that his respiratory condition improved in a supine position, but it worsened in a sitting position, and we diagnosed him as having platypnea-orthodeoxia syndrome. Transesophageal echocardiography showed right-to-left shunting of blood flow through an atrial septal defect. In the present case, reduced lung volume resulting from chronic hypersensitivity pneumonia possibly contributing to development of the platypnea-orthodeoxia syndrome.