A 3.5-year-old domestic long-hair cat was admitted to the veterinary hospital for routine procedures, including dental prophylaxis. The cat appeared clinically normal. General anesthesia was induced, and 30 minutes later, the pop-off valve was inadvertently left in a closed position. The cat developed pneumothorax, which was treated via thoracentesis and administration of oxygen. The condition resolved and the cat was released from the hospital to its owner. Barotrauma resulted because of high pressure in the anesthetic circuit. Barotrauma is a life-threatening complication of general anesthesia.