A 7-month-old castrated male Italian Greyhound, weighing 5.5 kg was presented with vomiting and acute onset of severe dyspnea. According to the owner, the dog had been otherwise healthy without any relevant history. The dog had vomited several hours before attending the veterinary clinic, and subsequently, developed severe dyspnea after vomiting. On initial examination by the referring veterinarian, the dog was tachypneic (50 breaths/min). A large amount of air accumulation was observed in the left pleural space by radiographic evaluations done by the referring veterinarian. Left-sided thoracocentesis was performed with evacuation of 1.5 L of air from the thoracic cavity. After thoracocentesis, the dog was referred to the Veterinary Medical Teaching Hospital as an emergency case. On physical examination, the dog was alert and responsive with normal mucous membrane color and normal capillary refill time. The dog had a rectal temperature of 38.2 °C, pulse rate of 160 beats/min and a respiratory rate of 60 breaths/min showing labored breathing. The auscultation revealed indistinct heart and lung sounds on the left side of the thorax, but normal auscultation on the right side. Plain thoracic radiographs revealed the accumulation of a large volume of air in the left pleural cavity inducing displacement of diaphragm and mediastinum, and a hyperlucent left cranial lung lobe. After emergency treatment including oxygen supplementation and thoracocentesis, computed tomography (CT) scanning was performed under general anesthesia and mechanical ventilation for further investigation. The CT images revealed the left-sided tension pneumothorax and emphysematous left cranial lung lobe with several bullae. On CT scanning images, the left cranial lung lobe appeared as a single lobe with a single lobar bronchus that was not divided into cranial and caudal parts. Based on these findings, CLE and a ruptured pulmonary bulla were the most likely cause of the tension pneumothorax. After CT examination, a left-sided thoracostomy tube was placed, and continuous suction with a three-bottle system providing 10 to 15 cm negative pressure was applied for 3 days because of the rapid accumulation of air. Since the amount of air in the thoracic cavity did not decrease despite continuous suction, a surgical treatment was decided upon to remove the affected left cranial lung lobe. The dog was premedicated with cefazolin (20 mg/kg intravenously [IV]), butorphanol (0.2 mg/kg IV), famotidine (0.5 mg/kg IV), and midazolam (3 mg/kg IV). After induction with propofol (4 mg/kg IV), the patient was intubated with an endotracheal tube and maintained with isoflurane (2%) in oxygen. Left fifth intercostal thoracotomy was performed in a routine fashion. The left cranial lung lobe appeared to be emphysematous with several small bullae and a large bulla, which was confirmed as the source of air leakage. A complete lobectomy of the left cranial lung lobe was performed using a thoracoabdominal stapler (DSTseries™ TA 30 mm Stapler, Covidien). The remaining left caudal lung lobe appeared to be collapsed, but its re-inflation was confirmed after positive end pressure ventilation. On inspection of the removed left cranial lung lobe, there was no division into cranial and caudal parts, but a tissue mass that was flat and less than 1 cm in size was attached to the hilum of the left lung lobe and located cranial to the left cranial lung lobe. Before closing the thoracotomy site, the thoracic cavity was filled with warm saline to detect any air leakage. A thoracostomy tube was placed and the intercostal thoracotomy was closed with 2-0 polydioxanone sutures around the ribs near the incision. Positive end pressure ventilation was maintained during the slow evacuation of the air from the thoracic cavity via the thoracostomy tube. The dog recovered smoothly from anesthesia without any complications. Postoperatively, the dog exhibited a normal condition without dyspnea, and air retention was not identified in the thoracic cavity on postoperative radiographs. The radiographs revealed adequate expansion of the collapsed left caudal lung lobe. For postoperative analgesia, a continuous rate infusion of fentanyl (0.004 mg/kg/h) and lidocaine (1.2 mg/kg/h) was administered for 24 h postoperatively, followed by oral carprofen (2.2 mg/kg) and tramadol (4 mg/kg) twice daily for 7 days. The dog was discharged on the fifth postoperative day after removal of the thoracostomy tube. During 16 months of follow-up, the dog stayed well without any respiratory or radiographic abnormalities. The excised left cranial lung lobe and tissue mass attached to it were histologically examined. The mass was identified as completely atelectatic lung tissue that was suspected to be the cranial part of the left cranial lung lobe (CrLtCr) based on its anatomical location. In the section, most alveoli had collapsed and bronchioles appeared to be somewhat dysplastic with normal columnar type epithelial cell lining. There were also areas of disorganized cartilage plate morphology and excessively smaller airways that may represent tertiary bronchi without adjacent cartilage plates. There was indications of hypertrophy of the medium-sized pulmonary arteries with vascular proliferation. The excised left cranial lung lobe, thought to be the caudal part of the left cranial lung lobe (CauLtCr), based on the anatomical location and shape, was characterized by the presence of emphysematous lung tissue, with marked ectasia of the alveolar lumens and terminal bronchioles and occasional formation of blebs and bulla. The classification of blebs and bullae was made by those location in the lung, that the bleb was found between the lung parenchyma and visceral pleura and the bulla was within the emphysematous parenchyma. Despite blebs is usually considered to be smaller than bullae, the diameters of the bleb and the bulla measured approximately 7 mm and 4 mm, respectively. This specimen also revealed that the cartilaginous plates lining the smaller or medium-sized bronchi appeared to be dysplastic and occasionally underdeveloped. According to the gross and histopathological findings, the excised pulmonary tissues were confirmed to have PH of CrLtCr and CLE of CauLtCr.