Three cases are presented of eventual diagnostic errors concerning the thymus. In the first case right aberrating lobe appeared radiologically as a tumour of posterior mediastinum. In the second case, the large thymic malleability, enabled it to invade medium mediastinum and to show an extraordinary variability in shape when in decubitus. Finally, a post surgical hematomata, in a patient affected by a cyanogen cardiopathy actually showed some problems of radiological diagnosis with the characteristic thymus regeneration.