Ectopic thymic tissue can present a diagnostic dilemma when it is located in the posterior mediastinum. The diagnosis can be made by awareness of it and bq use of computed tomography (CT) and magnetic resonance imaging (MRI). Rarely, ectopic thymus are reported to cause airway obstruction. In infants ectopic thymic tissue should also be considered in the differential diagnosis of secondary pneumonias and emphysema especially located in the upper lung zones. (C) Elsevier Science Inc., 1999.