Cytokeratin 19 (CK19) is an acidic protein of 40 kDa that is part of the cytoskeleton of epithelial cells and is highly expressed by differentiated thyroid carcinomas, mainly of the papillary subtype. The soluble fragments of CK19 (Cyfra 21.1) can be measured by immunometric assays employing specific monoclonal antibodies. The present study was planned to assess the serum expression of Cyfra 21.1 in patients with benign thyroid nodules and thyroid malignancies. We enrolled 135 patients with histologically proven benign thyroid nodules (n=79) and thyroid carcinomas (n=56). No differences were found in serum Cyfra 21.1 levels between patients with benign nodules and patients with carcinomas. When thyroid malignancies were subdivided according to tumor histology, serum Cyfra 21.1 increased significantly from classical differentiated thyroid carcinomas (papillary or follicular) to less differentiated or undifferentiated carcinomas (poorly differentiated or anaplastic). CK19 release into the bloodstream is strongly related to the apoptotic pathway, and particularly to hyperproliferation-related apoptosis. These pathways characterized anaplastic and poorly differentiated thyroid carcinoma but not classical forms of differentiated thyroid carcinoma. Consequently, Cyfra 21.1 may be regarded as a circulating marker of poorly differentiated and anaplastic thyroid carcinoma. Additionally, a role of Cyfra 21.1 as a dedifferentiation marker in patients with classical differentiated thyroid carcinomas may be postulated and should be explored by further focused studies.