We examined the expression of pancreatic secretory trypsin inhibitor (PSTI) in colorectal cancer by immunohistochemical staining using an anti-PSTI antiserum, an in situ hybridisation technique utilising sulphonated PSTI cDNA probe, and a Northern blot hybridisation method, using a 32P-labelled PSTI cDNA probe. Immunohistochemically, PSTI was detected in 80 of 95 (84%) colorectal cancer cases. Analyses with in situ hybridisation as well as Northern blot hybridisation demonstrated PSTI mRNAs in immunohistochemically positive cases, showing PSTI could be produced in colorectal cancerous cells. Histologically well or moderately differentiated adenocarcinoma showed higher incidence of PSTI immunoreactivity than the other types. Furthermore, the intensity of the immunohistochemical staining for PSTI increased the more cases advanced, particularly in regard to depth of invasion and tumour size. Thus, PSTI expression is widespread in colorectal cancer, and occurs more commonly in advanced cases. Considering the suggestion that PSTI is a growth-stimulating factor as an well as inhibitor to proteolytic proteinase, the present findings may indicate that PSTI expressed in colorectal cancerous cells may play a role possibly closely associated with tumour development.