Lymphangiomas are rare and correspond to 0.7% to 4.0% of mediastinal tumors, and isolated mediastinal location occurs in 1% of cases. They are benign tumors that originate from a congenital malformation of the lymphatic vessels and are diagnosed more frequently in children less than 2 years of age. Chylous ascites is a clinical manifestation of thoracic duct lymphangioma and is composed of lymph accumulation caused by dilation of this lymphatic channel. It appears milky in the peritoneal cavity, containing triglyceride levels higher than 200 mg/dl. We report the case of a young patient with chylous ascites and lymphangioma of the thoracic duct, who was conservatively treated with octreotide and a low-fat diet with medium-chain triglycerides.