Cytokeratins are epithelial markers whose expression is not lost during malignant transformation. CYFRA 21–1 is a cytokeratin-19 fragment that is soluble in serum and can be used as circulating tumor marker. Although expressed in all body tissues, its major occurrence is in the lung, particulary in lung cancer tissues. CYFRA 21–1 is a sensitive and specific tumor marker of non-small-cell lung cancer (NSCLC), especially of squamous cell subtype (1,2,3). It also reflects the extent of the disease and has an independent prognostic role along with performance status and disease stage in NSCLC (4,5,6). In addition, detection of serum CYFRA 21–1 allows for identification of high risk patients that may benefit from adjuvant chemotherapy (7), and enables the early detection of progressive disease in recurrent NSCLC (8). Additionally, CYFRA 21–1 has been described as a useful marker for esophageal squamous cell carcinoma (9) and for therapy monitoring of bladder cancer (10).