An 8 month old boxer dog was referred for management of acute spontaneous pneumothorax. CT showed multiple variably sized bullae in the right middle lung lobe, and partial or complete atelectasis of all remaining lung lobes. This was assumed to be a case of congenital lobar emphysema (CLE). The dog was anaesthetised for surgery but died from cardiopulmonary arrest during presurgical thoracocentesis. Postmortem examination confirmed that ruptured bullae in the right middle lung lobe were responsible for the dog's pneumothorax and death. In addition, a malformation of the right subclavian artery was present. Histologically, the right middle lung lobe and primary bronchus were malformed, with microscopic changes resembling those of human congenital pulmonary airway malformation (CPAM), rather than CLE. CPAM has not been reported in the veterinary literature and should be considered in the differential diagnosis for spontaneous pneumothorax in dogs.