The development of a widened mediastinum in patients with neoplastic disease frequently indicates tumor recurrence or spread. Seven cases of mediastinal widening in children with a variety of neoplasms are described wherein the etiology is either proven or strongly suspected to be benign thymic enlargement. Awareness of this benign etiology for mediastinal widening may avert a diagnostic surgical procedure. A steroid trial in certain specific cases may assist in differentiating benign from malignant etiologies.