Thymomas are exceedingly rare in the first 20 years of life, and only a few well-documented cases are present in the literature. Thymomas in adults are commonly associated with other diseases, the most frequent being myasthenia gravis (MG). However, this association has been reported not to occur in childhood. The authors report the case of a 4-year and 10-month-old girl who presented with clinical evidence of MG before thoracotomy and who developed florid MG after thoracotomy for removal of an anterior mediastinal mass later documented to be a thymoma.