Chylothorax is an uncommon, frequently idiopathic condition in cats and dogs, characterized by the accumulation of chylous (lymphatic) fluid within the pleural space. Treatment options vary considerably and success has been limited particularly with cats. Failure to resolve can result in an uncommon but severe and often fatal progression to restrictive pleuritis. Management techniques for this latter condition are further limited and prognosis is often poor. In the following case study, a four year old female spayed domestic shorthair cat presented for a three week history of pleural effusion. Clinical signs were limited to episodes of mild lethargy and mild to moderate tachypnea without noted dyspnea. The patient had responded well, clinically, to frequent therapeutic thoracocentesis by the referring veterinarian since onset, and no obvious progression in clinical signs had since been noted. A diagnosis of chylothorax was made, with strong suspicion of an intrathoracic mass based on imaging. The patient was referred to Cornell University Hospital for Animals for potential surgical excision of this suspected mass. Pleural fluid analysis was used to confirm the diagnosis of chylothorax, and evidence of a severe, chronic, restrictive pleuritis was demonstrated by both thoracic radiographs and computed tomography, however presence of a thoracic mass on imaging was disputable. Exploratory thoracotomy was pursued despite a grave prognosis. The summary of this case is followed by a brief review of chylothorax and associated restrictive pleuritis with particular mention of treatment options.