We report a rare case of intrathoracic lymphangiomatosis associated with chylothorax in a 3-year-old boy. The patient had been healthy until he had chickenpox followed by continued dyspnea. The biopsy specimen showed proliferating lymphatic channels & spindle cells in the lung tissue. The boy died of respiratory failure despite conservative treatment and surgical treatment with pleurodesis. An autopsy was performed. Pleura and pericardium showed severe adhesion to the right lung parenchyma. The cut surface of lung showed thickened interlobular septum with honeycomb-appearance. Histologically, extensive intercommunicating and anastomosing endothelial-lined lymphatic channels were noted along the visceral and parietal pleura, pericardium, around the great vessels, and in the anterior mediastinum over the thymus. These lymphatic channels infiltrated into the pulmonary parenchyme along the bronchovascular bundles. There were scattered areas of spindle cell proliferation with extravasation of RBCs mimicking Kaposi's sarcoma. Histologic diagnosis and differential diagnosis on biopsy materials can be difficult to establish but awareness of the intrathoracic lymphangiomatosis and its various clinical presentation may be helpful for diagnosis.