Thymic hyperplasia associated with Graves’ disease is rarely reported in children, although it is not uncommon in adults. Occasionally, an enlarged thymus presents as an anterior mediastinal mass on a radiographic examination. Such patients often undergo invasive procedures such as a thymus biopsy or thymectomy because of suspected malignancy. However, an enlarged thymus with Graves’ disease is known to shrink after treatment with antithyroid drugs. Therefore, recognition of this benign course would avoid unnecessary surgical resection. This report presents the case of a 10-yr-old boy with Graves’ disease complicated with an anterior mediastinal mass. Computed tomography showed a homogenous mass with no invasion into the surrounding tissue. A gallium-67 scintigraphy showed no abnormal uptake. Shrinkage of the mass after treatment with an antithyroid drug (methyl-mercaptoimidazole) supported the diagnosis of thymic hyperplasia with Graves’ disease. This case report illustrates two important points. First, pediatricians should be aware that thymic hyperplasia can coexist with Graves’ disease, even in children. Second, close radiographic assessment would support a diagnosis of thymic hyperplasia and eliminate invasive diagnostic procedures.