Anterior mediastinal and adenoid masses in children after cessation of chemotherapy for malignant disease open cause a diagnostic problem. Differential diagnosis of thymic enlargement and adenoid hyperplasia from recurrence frequently poses a challenge both for the radiologist and the physician. In this study the authors evaluated, 491 patients with different malignant tumors for thymic and adenoid hyperplasia. Thymic hyperplasia was seen in 18 patients (5 Hodgkin disease (HD), 5 non-Hodgkin lymphoma (NHL), 4 Wilms tumor, 2 germ cell tumor, 1 Ewing sarcoma, and 1 neuroblastoma), only adenotonsillar hyperplasia was seen in 6 patients, all with NHL, and both thymic and adenotonsillar hyperplasia were seen in 3 patients (1 HD, 2 NHL). In 5 patients, adenoid hyperplasia was proven by biopsy; 1 patient. underwent to adenoidectomy. Their histopathologic investigation showed polyclonal follicular hyperplasia. The authors recommend that patients with thymic and/or adenotonsillar enlargement after successful treatment of their primary malignancy should be evaluated cautiously before an invasive procedure is planned.