The objective of this study was to develop a method using liquid chromatography with tandem mass spectrometric detection for the absolute quantification of the small cell lung cancer biomarker ProGRP in human serum, using its tryptic signature peptide NLLGLIEAK. The samples were precipitated for most of its proteins using acetonitrile prior to tryptic digestion. Further sample clean-up and enrichment was achieved by the use of an on-line restricted access media column, followed by separation on a BioBasic C8 column. Detection and quantification was carried out by operating a triple quadrupole MS in the selected reaction monitoring mode. This setup allowed analysis of realistic samples and detections limits in human serum of 150 pg ProGRP on column. Using an internal standard derived from the parent ProGRP after acetylation of the lysine side chain allowed better quantification through variation correction in all sample pretreatment steps, trypsination included.