Differential diagnosis of suspicious lung masses is essential for the selection of the appropriate therapy strategy. While non-small cell lung cancer (NSCLC) in early stages and single lung metastases from other cancers mostly are resected by surgery, late stage NSCLC, small cell lung cancers (SCLC) and multiple lung metastases are treated by systemic chemo- and/or radiotherapeutic approaches. In many patients, biopsies for the histopathological subtyping can not be taken due to multimorbidity and instable clinical conditions of the patient or unfavourable localisation of the tumor. In addition, heterogeneity of lung tumors may imply the presence of different malignant cell types in one suspicious lesion. As tumor-related biomarkers in blood reflect the biochemical properties of cancer cells, their release or non-release may be helpful to support the clinical decision making. This review summarizes the current knowledge about the potential and the role of serum-based biomarkers for the differential diagnosis of lung cancer which is also mirrored in the new recommendations of the National Academy of Clinical Biochemistry (NACB).