The CT examination of 36 patients with masses in the thymus (three thymus hyperplasias, 33 thymomas) were evaluated retrospectively. Three tumours in atypical positions posed problems in differential diagnosis from bronchial carcinomas and two cases with extensive pleural metastases had to be differentiated from pleural mesotheliomas. There were no certain morphological features on CT which indicated whether the mass was malignant. Eleven patients with thymomas had metastases; in seven of these they were extra-thoracic, with preference for supraclavicular lymph nodes, adrenals and the axial skeleton. Tumour progression and distant metastases were found particularly in malignant thymomas of the epithelial type. Calcification in a thymoma tends to indicate that it is malignant.