Thymic tissue can be encountered in about 20 % of mediastinal teratomas. It is found located at the periphery of the teratoma mass. Although rare, mediastinal teratoma arising within the thymus can be seen. It has not been defined yet whether the thymus is an organ simply attached to the teratoma or whether the teratoma arises from the thymus. The authors present here two illustrative cases of mediastinal teratoma, one raised within the thymus and the other closely attached to the thymic remnant, to discuss the cellular origin of mediastinal teratomas.