True thymic hyperplasia in an infant.

True thymic hyperplasia is a very rare entity. We present an instance of idiopathic true massive thymic hyperplasia in a 9-month-old girl with a very large left-sided mediastinal mass noted on diagnostic imaging. Percutaneous biopsy revealed normal thymic tissue. Steroids were administered with no response. Surgery may be required in patients with respiratory distress unresponsive to steroids.