Paraneoplastic neurological syndromes (PNS) are autoimmune neurodegenerative diseases that develop as a result of the cross-reactivity of the tumor-specific immune effectors with neurons of central and peripheral nervous systems. So-called onconeural antibodies, which are detected in sera of PNS patients, are not only crucial diagnostic markers of PNS and associated tumors, but also have a considerable potential in the serological diagnosis of cancer as a whole. In this review we discuss the role of onconeural antibodies in serological diagnosis of PNS and associated tumors as well as their potential in diagnosis and prognostication of a broad spectrum of malignant tumors.