In a group of seventy patients with small cell lung cancer the prognostic value of serum tumour markers was determined. Thymidine kinase (TK), tissue polypeptide antigen (TPA) and lactate dehydrogenase (LDH) but not neuron specific enolase (NSE) correlated significantly with survival. Since all markers were strongly interrelated with each other and with the extent of disease, the combined determination of TK, TPA and LDH or the combination of disease extent and a marker yielded no more prognostic information than a single measurement of one of these variables.