In a retrospective study, 108 thoracic CT-scans of 18 patients aged 5-25 who had received cytotoxic treatment for solid tumors were evaluated. The thymic size decreased in 15/17 patients during treatment and increased in 14/15 after the end of therapy. In 5/5 patients, the change could also be detected with relapse therapy. These changes have to be considered in the evaluation of a mediastinal mass suspected of metastasis during and following cytotoxic therapy.