Cervicomediastinal cystic hygroma is a rarely encountered pathological entity. Gross and Hurwitt1 found 19 cases in the literature and added 3 of their own. The lesion in the case to be reported appears to be the second mediastinal hygroma associated with chylopericardium and the third case of spontaneous chylopericardium recorded. The hygromatous cyst, typically, is a lobulated, thin-walled translucent sac lined by flattened endothelium. The lobules are usually separated by fibrous septa, but there may be free communication between them. The contained fluid is thin, clear, and faintly yellow. The lobules may extend between muscle bundles, nerve fibers, and other anatomical structures in the involved area. Goetsch2 postulated that outgrowths or cords of endothelial cells insinuate themselves between the anatomical structures and by collection of fluid force apart the walls of the cords, developing lumina, thereby distorting the surrounding anatomy. Hygromas tend to develop in the neck and,