Hyperplasia of the thymus, an uncommon development following chemotherapy, raises doubt concerning possible tumor recurrence. We report a case of thymic hyperplasia in a 12-year-old girl who was given chemotherapy for a cervicomediastinal lymphoma. Three months after treatment end, dry cough and an enlargered mediastinum suggested recurrence, but histological examination of tissue biopsy demonstrated benign reactive hyperplasia. Outcome was favorable without recurrence after corticosteroid therapy. According to the literature, thymic hyperplasia is neither tumor nor treatment specific. It generally occurs in children but may be observed in young adults. Hyperplasia of the thymus following chemotherapy would appear to be a good prognosis factor.