Superior herniation of normal mediastinal thymus into the anterior neck is a rare cause of neck masses in children. It is defined as intermittent migration of the broadest part of the normal thymus out of the thorax into the suprasternal region during Valsalva maneuver with an increase in the intrathoracic pressure. The fact that the mass apparent only during Valsalva maneuver and typical ultrasound characteristics usually allow the diagnosis but computerized tomography scan or magnetic resonance imaging is necessary to assess the extent of the mass. We report the first and the only siblings with the most dramatic degree of superior herniation of normal mediastinal thymus. We discuss the findings of imaging and the differential diagnosis. We try to remind this entity to avoid unnecessary biopsy or surgery and their potential risk of altering immune function. Copyright © 2008 by Lippincott Williams & Wilkins.