Chylothorax is the accumulation of lymphatic fluid in the pleural space. Lymph may leak from intact lymphatics with lymphangiectasia or dilated mediastinal and pleural lymphatics. The present study report a case of chylothorax presented to the Veterinary Hospital at the University of Londrina (HV-UEL) in July 2008. A 5-year-old intact male Mastiff dog was evaluated at the HV-UEL because of a cystine urolith recurrence. At the clinical examination the dog was in mild respiratory distress, manifested by openmouthed breathing, increased inspiratory and expiratory effort. The radiography revealed pleural effusion and a thoracentesis was performed to remove the fluid. CBC and serum biochemical analyses were within reference range. Cytologic analysis of the fluid sample revealed a white, opaque fluid that contained 5.5 g/dL of protein and 144 nucleated cells/mm³ (59% lymphocytes, 16% neutrophils, and 25% macrophages). The pleural fluid triglyceride and cholesterol concentrations were 1,226 and 30 mg/dL, respectively, compared with concentrations of 64 and 46 mg/dL respectively, in serum. These values were consistent with chylous pleural effusion. Result of serologic test for Dirofilaria immitis and microbial culture of pleural fluid were negative. Echocardiographic imaging, thoracic radiography and ultrasonography did not reveal an underlying cause and the dog had history of possible trauma. The dog was administered amoxicillin clavulanic acid (20mg/kg, PO, BID) and rutin (50mg/kg, PO, TID) and fed a low-triglyceride diet. Over the ensuing 5 weeks, the dog had recurrent episodes of tachypnea and dyspnea, necessitating thoracocentesis to decompress the pleural space on 5 occasions. Serial radiographic studies revealed diminishing volumes of pleural effusion and analysis of thoracic radiographs taken 4 months after medical treatment revealed complete resolution of the pleural effusion. Chylothorax may be idiopathic, but several causes have been implicated including cardiomyopathy, thoracic and mediastinal neoplasia, heartworm disease, congenital lymphocytic defects, trauma, chronic vomiting or cough, rupture of lymphatic vessels, diaphragmatic hernia, lung lobe torsion and venous thrombi. In the present case, lymphography would be useful for evaluation the integrity of thoracic duct and find the underlying cause, but could not be performed because the owner did not authorize the invasive procedure. We concluded that the cause of chylothorax was the trauma because of the history, complete resolution and discard of others causes.