Pneumothorax is defined as the occurrence of air in the pleural space. From a clinical standpoint, pneumothorax can be classified as spon taneous (without an obvious triggering factor) or nonspontaneous. Primary spontaneous pneumothorax (PSP) is defined as the sponta neous presence of air in the pleural space in patients without clinically apparent lung disease. We present a case of a 26-year old man who reported chest pain at rest. A standard chest x-ray (CXR) picture on inspiration did not reveal any severe pathology, but a second imaging on expiration showed a large pneumothorax. In this case, the pneumothorax would have been undetected if only the inspiratory CXR was used. Lung ultrasonography (USG) can be used to diagnose radio-occult pneumothoraxes independent of the respiratory phase of the patient.