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The objective of this study is to report the development of thymic enlargement in adults, mainly associated with chemotherapy for malignancy. The typical CT features of this phenomenon are described. The clinical data and CT studies of 13 adult patients with newly appearing thymic enlargement on CT were reviewed. These patients were followed-up mainly for malignancy. Further follow-up CTs were studied when available. Medical records were reviewed as to the primary disease, its medical treatment and the time of initial appearance of the enlarged thymus in relation to treatment. The study group included 13 adult patients, 12 with malignant disease and one with a slowly resolving pneumonia. The enlarged thymus appeared as a triangular, arrowhead-shaped structure, with a bilobed configuration and convex borders. Density measurements were consistent with homogeneous soft tissue. Location was in the anterior mediastinum, in the normal site of the thymus. In nine patients, follow-up studies were available. The observation period ranged from 5 months to 8 years from the initial appearance of the enlarged thymus. In five of the nine patients, the thymic enlargement resolved after 1-4.5 years. In four of the nine patients, the thymus remained enlarged during a follow-up ranging from 5 months to 2.5 years. Thymic enlargement, while a rare phenomenon in adults, may occur, mainly after chemotherapy. This phenomenon should be included in the differential diagnosis of a soft tissue mass appearing in the anterior mediastinum on follow-up CT in adult patients particularly following treatment for malignancy.