Chylothorax is a rare but complex disorder in domestic animals. Etiologies include neoplasia, fungal infections, heartworm infestation, cardiac disease, thrombosis of the cranial vena cava, and congenital anomaly of the thoracic duct. Most cases of chylothorax in dogs and cats are idiopathic. Positive contrast lymphangiography on dogs and cats with chylothorax consistently reveals extensive lymphangiectasia of mediastinal and pleural lymphatics. Reported treatment modalities for chylothorax in animals include removal of the etiologic agent, such as a mediastinal tumor, thoracic duct ligation, and implantation of active or passive drainage devices such as a pleuroperitoneal shunt. Thoracic duct ligation has been most successful in our experience, but continued study is needed since treatment failures are common.