Six cases of nontraumatic chylothorax secondary to filariasis, postinflammatory thoracic duct obstruction, idiopathic lymphangiectasis with lymph node hypoplasia, lymphangioma and lymphoma (2 cases) are reported. All cases were studied by lymphangiography, which provided considerable diagnostic information. In those cases which are undiagnosed at the time the chylothorax is discovered, a lymphangiogram should be carried out prior to any therapeutic measures.