Hydatid cyst is a zoonotic cycle parasitic infection caused by Echinococcus granulosus tapeworm, whose larvae infect man as an intermediate host. It is one of the most serious lung diseases caused by helminths. It is rare to find it in the mediastinum; < 1%. The primitive, or seemingly primitive, location is due to the parasite in the cellular tissue of the mediastinum. The authors report the case of a sixteen year old male, admitted to the E.R. for left posterior chest pain which had spread to the upper left limb.