A 5-year-old male castrated Golden Retriever presented to the Cornell University Hospital for Animals Emergency Service on January 7, 2012 for evaluation of acute pneumothorax diagnosed by his referring veterinarian. On physical exam, the dog was dyspneic, tachypneic, and tachycardic and had decreased lung soungs in the dorsal lung fields bilaterally. Diagnostics performed revealed dehydration, respiratory alkalosis, and hypoxemia. A lung bulla located in the left cranial lung lobe was identified on CT scan; therefore the patient was immediately taken to surgery for a left sided thoracotomy and left cranial lung lobectomy. The patient recovered well from surgery, with dramatic improvement in lung sounds, respiratory rate, and respiratory effort over the next 48 hours. This report will discuss the clinical history, diagnosis, pathophysiology, and treatment of spontaneous pneumothorax.