We describe two cases of encapsulated thyroid tumours which displayed the classic morphological features of hyalinizing trabecular adenoma. In addition, both were characterized by focal invasion of the capsule and of thin-walled capsular blood vessels. Positive immunohistochemical staining of tumour cells for thyroglobulin and negative staining for calcitonin, chromogranin and CEA allowed distinction from medullary carcinoma. Electronmicroscopy revealed groups of tumour cells, surrounded by abundant basement membrane type material. Occasional tumour cells contained abundant cytoplasmic intermediate filaments. A flow cytometric analysis revealed one tumour to have a diploid DNA pattern and the other to be DNA aneuploid. These cases illustrate that a malignant variant of hyalinizing trabecular adenoma, namely hyalinizing trabecular carcinoma, exists. Hyalinizing trabecular tumours of the thyroid should not be considered uniformly benign lesions. As with follicular neoplasms, multiple sections from the capsule should be examined histologically in order to assess the presence or absence of capsular and/or vascular invasion.