Expression of the human pancreatic secretory trypsin inhibitor (PSTI) gene was examined in 24 cases of neoplastic tissues by Northern blot analyses. In three cases of lung adenocarcinoma and one case of sigmoid colon polyp, we detected transcripts which hybridized to the human PSTI cDNA probe. cDNA libraries were constructed using mRNAs of the two PSTI-positive tumor tissues. Two PSTI cDNA clones were obtained from each sample. Sequencing analyses showed that they were completely identical with that of pancreatic PSTI cDNA which had been reported [(1985) Biochem. Biophys. Res. Commun. 132, 605-612]. Southern blot analyses showed that the elevated expression of PSTI in neoplastic tissues was accompanied by neither PSTI gene amplification nor rearrangements.