The concurrent finding of two thyroid carcinomas is rare. The authors present two patients with the concurrent occurrence of two tumours. In the first case--a 51-year-old man--the authors found a medullary carcinoma in the right lobe and a medullary papillary carcinoma in the left thyroid lobe. The medullary papillary carcinoma metastatized into the mediastinal lymph nodes and into the mediastinal tissues in the area above the tracheal bifurcation. From the morphological and immunohistological aspect in the primary tumour of the left lobe as well as in the mediastinum two different clones of tumour cells were found (the medullary component of the tumour responded positively to antibodies against calcitonin and the papillary component to serum against thyroglobulin. The patient was treated by radiotherapy. The tumour progressed however markedly in its medullary component and the patient died one and a half year after the first symptoms from generalization of the tumour. The second patient, a woman, had an occult papillary carcinoma in the right lobe and in the left lobe a of a medullary papillary carcinoma (follicular variant). This type of tumour also metastatized into a cervical lymph node in the area of the left lobe. The patient was treated by radioiodine. Nine months after bilateral thyroidectomy she has no signs of progression of the tumour.