A 21-month-old Japanese boy was admitted with cough and hypoxemia. Chest X-ray showed massive right pleural effusion, which consisted of chyle. Computed tomography showed poor contrast area at superior and anterior mediastinum. Magnetic resonance imaging showed granular T2-low area at the same area. Lymphangioscintigraphy revealed a hot spot at superior mediastinum. These findings lead us to diagnose as mediastinal lymphangioma accompanied with chylothorax. Noninvasive treatments including total parenteral nutrition, administration of octreotide and sclerotherapy were tried, but all of them proved to be ineffective. Transfusions of blood products were frequently needed during these therapies. On the 48th hospital day, the mediastinal tumor and the thymus were excised through a median sternotomy. A leakage point of lymph into the intrathoracic space was not found, in spite of preoperative administration of milk with dye. Since the pleural effusion had continued to be drained, pleuroperitoneal shunt was placed on the 90th hospital day. The shunting amount continued to decline soon after the shunting, and had been under 10 ml/day since the 142nd day. The shunt was removed on the 148th day. There has been no reaccumulation of the pleural effusion and no recurrence of the mediastinal tumor for 1 year of observation.