An 8-year-old boy who presented with a mediastinal mass, pulmonary infiltrates, and disseminated intravascular coagulation was diagnosed with lymphangiomatosis. Despite medical management, he developed multiple organ failure and died. The authors discuss the diagnostic findings, medical management, and pathology and review 52 additional cases of thoracic lymphangiomatosis from the literature. Patients presented with chylothorax (49%), a mass (47%), pulmonary infiltrates (45%), bone lesions (39%), splenic lesions (19%), cervical involvement (15%), disseminated intravascular coagulation (9%), and skin involvement (7%). Children (<16 years) had a worse prognosis than older patients (39% vs. 0% mortality). All patients who died had either parenchymal lung involvement or pleural effusion. Thoracic lymphangiomatosis should be included in the differential diagnosis of a mediastinal mass with pulmonary findings.