Two medullary carcinomas of the thyroid (MCT) with relatively unusual patterns are reported. The first was an aggressive tumour which occurred in a 66-year-old man and displayed in most areas follicular structures. The second tumour occurred in a 36-year-old woman, followed a benign course and showed papillary infoldings lined by multilayered neoplastic cells. The search for thyroglobulin yielded negative results whereas calcitonin immunoreactivity could be found in most neoplastic cells of both tumours. The diagnosis of MCT was further substantiated by the presence of scarce amyloid deposits and typical neuro-secretory granules by electron microscopy. These cases demonstrate once more that follicular and papillary structures can be a prominent feature of some MCTs reinforcing therefore the major role of immunocytochemistry in the differential diagnosis of thyroid carcinomas. Papillary MCT seems to carry a good prognosis in contrast to follicular MCT if one takes into account the follow-up data of the present cases together with those of similar cases reported in the literature; the whole series is nevertheless too small to allow for definite conclusions on this matter.