Serological screening techniques, such as SEREX and AMIDA, use tumour-specific antibodies in the blood of patients to identify tumour-associated antigens. Using AMIDA, we have recently reported the identification of cytokeratin 8 (CK8) as a new tumour antigen and the presence of elevated serum levels of CK8-specific antibodies (CK8-Abs) in patients with head and neck cancer (HNC). The translation of such a biological observation into a reliable biomarker for clinical diagnosis is a major challenge across the border of biological and clinical research. A prerequisite for a valuable biomarker is specificity, ie. its restriction to a defined disease. Whether the serum levels of CK8-Abs are elevated in patients suffering from other diseases besides HNC was evaluated in this study. A panel of 749 serum samples from patients with carcinomas and benign diseases of different localisations, autoimmune diseases and infections, as well as from clinically healthy donors, was tested. In total, 3.2 times more patients with benign disease (16%) and 2.4 times more patients with carcinomas (12%) showed specific CK8 serum reactivity compared to the healthy donors. However, reactivity was not associated with the kind of disease, i.e. malignant or benign disease, but rather with the disease localisation. Thus CK8-Abs on their own do not allow for the detection of carcinoma of a defined localisation or for the differentiation between carcinoma and benign diseases.