Personal experience with small cell thyroid tumours is reviewed in the light of recent developments in diagnosis and treatment. All cases were examined by means of immunohistochemical investigation of the lymphocytic and epithelial antigens. In 7 cases in which the production of lymphocytic antigens was confirmed, a primary lymphoma of the thyroid was diagnosed. Immunohistochemical studies of the lymphocytic and epithelial antigens are essential in all small cell thyroid tumours, in order to differentiate between small cell anaplastic carcinomas and thyroid lymphomas. This differentiation is indispensable for the correct choice of treatment and an accurate prognostic assessment. In the case of lymphomas, combined surgical and radiation treatment adjusted to the clinical stage of the tumour is recommended.