A ten-month old male castrated Bull Mastiff presented to the Cornell University Hospital for Animals Emergency Service for evaluation of acute respiratory distress. Physical exam, thoracic radiography and thoracocentesis identified pleural effusion. Chylothorax was diagnosed by cytology, triglyceride levels higher than serum and a cholesterol/triglyceride ratio <1. No underlying cause for increased lymphatic and venous pressure or occlusion of lymphatic and venous drainage was found using thoracic and abdominal ultrasonography, echocardiography, thoracic radiography, heartworm serology, CBC and serum chemistry analysis. Medical management in the form of a low fat diet, thoracocentesis and Rutin was instituted. After two additional emergency presentations following medical management, the owners opted for surgery. Thoracic duct ligation, partial pericardiectomy and omentalization were performed with subsequent decrease in effusion volume and change to inflammatory nature. Diaphragmatic hernia from the omentalization site developed post-surgically and was surgically repaired. The chylous effusion had not recurred five months post surgery.