Diffuse lymphangiomatosis is a very rare congenital disease, characterized by diffuse or multifocal lymphangioma in the skeletal tissue, spleen, liver, mediastinum, and/or lung. The prognosis is usually poor, especially for children with thoracic lesion, and treatments for the disease are controversial. The authors report a 9-year-old boy with diffuse lymphangiomatosis involving the thorax with pleural effusions, the spleen, and systemic bone. The patient was treated with pegylated interferon alfa-2b, and achieved good clinical and radiological improvement.