PURPOSE: To evaluate the radiologic findings and clinical feasibility of thymic enlargement in patients with hyperthyroidism. MATERIALS AND METHODS: Seven patients with hyperthyroidism and anterior mediastinal bulging revealed by chest radiogaphy were evaluated. The CT findings were analyzed with regard to the shape of the anterior mediastinal mass, surrounding infiltration, and enlargement of mediastinal lymph nodes. Whether or not tumor markers (alpha-fetoprotein, beta-human chorionic gonadotrophin, and chorionic embryonic antigen) showed increased levels was determined, and the size and thickness of the anterior mediastinal mass were measured and compared with previously described age-matched thymus data. In addition, changes in the thyroid gland were evaluated. RESULTS: In all seven patients, anterior mediastinal masses were bi-lobed, with no surrounding infiltration or enlarged mediastinal lymph node, and tumor marker levels showed no increase. The masses were therefore considered to be thymus. In six patients, the size of the thymus exceeded two upper standard deviations of mean value and in one patient, it was smaller than this. In three patients, PCNB (percutaneous needle biopsy) revealed normal thymic tissue and in two, follow-up chest PA demonstrated no interval change. CT showed that in three patients, the thyroid glands were diffusely enlarged. CONCLUSION: In patients with hyperthyroidism, an anterior mediastinal mass seen on chest radiographs was due to thymic enlargement. The recognition of CT findings of thymic enlargement in such patients may avoid unnecessary biopsy.