Two cases of massive thymic hyperplasia (MTH) are reported in boys 11 and 14 years of age. Each was detected on routine chest roentgenogram as a large anterior mediastinal mass. Total thymectomy was successfully performed and the two patients were alive and well 9 and 17 years later. The thymectomy specimens weighed 324 and 490 gm, respectively, and had uniform histologic anatomy without lymphoid nodules. Morphometric study documented the massive increase in thymic parenchyma (i.e., true hyperplasia) with retention of a normal proportion of connective tissue expected for age. Since thymic size and structure normally become modified with age, it is essential to consult standard reference data when evaluating hypertrophy and hyperplasia. Gross and histologic features distinguishing MTH from childhood thymoma are discussed.