A case of massive true thymic hyperplasia with non-Hodgkin's lymphoma of the mediastinum is reported in a 14-year-old boy. Computed tomographic scan of the chest showed a mass of the anterior mediastinum and conspicuous swelling of the lymph nodes in the upper and lower mediastinum. They were grossly resected. The tumor of the anterior mediastinum was histologically diagnosed as true thymic hyperplasia. All of the mediastinal lymph nodes were diagnosed as non-Hodgkin's lymphoma, diffuse, mixed small and large cell type.