Medical records of 34 dogs and 16 cats undergoing surgical repair of diaphragmatic hernia of >2 weeks' duration were reviewed, and long-term follow-up information was obtained. The most common clinical signs were dyspnea and vomiting; however, many of the animals were presented for nonspecific signs such as anorexia, lethargy, and weight loss. Thoracic radiographs revealed evidence of diaphragmatic hernia in only 66% of the animals, and additional imaging tests were often needed to confirm the diagnosis. Thirty-six hernias were repaired through a midline laparotomy; 14 required a median sternotomy combined with a laparotomy. In 14 animals, division of mature adhesions of the lungs or diaphragm to the herniated organs was necessary to permit reduction of the hernia. Fourteen animals required resection of portions of the lungs, liver, or intestine. All hernias were sutured primarily without the use of tissue flaps or mesh implants. Twenty-one of the animals developed transient complications in the postoperative period; the most common of these was pneumothorax. The mortality rate was 14%. Thirty-four (79%) of the animals that were discharged from the hospital had complete resolution of clinical signs, and none developed evidence of recurrent diaphragmatic hernia during the follow-up period. Nine were lost to follow-up.