Small cell lung cancers are neuroendocrine tumours and therefore produce a lot of peptide hormones (calcitonin, ACTH, ADH), as well as the neuropeptide chromogranin A, which are all useful tumour markers. Furthermore, the tumour-associated antigens CEA and TPA, as well as the enzymes neuron specific enolase (NSE) and creatine kinase BB are used as markers in small cell lung cancer. At present, NSE appears to be the best marker for small cell lung cancer; elevated serum NSE levels are found in 65 to 85% of the patients. The serum level of the tumour markers is related to the stage of the tumour. When tumour regression occurs following therapy, elevated pretreatment levels decrease to the normal range. If the marker level increases again, tumour progression is indicated and this can be an early and sensitive sign denoting recurrence. Metastases in the central nervous system can be detected early by marker determination in the cerebrospinal fluid. At present, CEA appears to be the most valuable tumour marker for non-small cell lung cancer, but TPA may also be a useful marker.