This study was aimed at assessing the significance of pS2 immunostaining in colorectal adenocarcinoma and adjacent tissue. Paraffin sections of 63 surgically resected colorectal adenocarcinomas were stained with a pS2 specific monoclonal antibody using the avidin-biotin complex immunoperoxidase technique. Several tumor sections also included adjacent nonneoplastic mucosa. Six tubular adenomas and five hyperplasic polyps found in the resected bowels were also examined. Focal staining for pS2 was seen in 32 tumors (51%). In all but one case, less than 10% of the tumor cells were stained. pS2 staining was more common in right-sided than in left-sided tumors (p < 0.05), and was more prevalent in Dukes C than combined Dukes A and B tumors (p < 0.05). No significant relationship was found between pS2 positivity and the degree of tumor differentiation, patients' sex, or outcome of the disease as judged by the development of recurrence or metastasis. Strong pS2 positivity was seen in hyperplastic polyps and in nonneoplastic mucosa adjacent to many tumors. Tubular adenomas were either negative or showed focal superficial staining. It is concluded that pS2 immunostaining in colorectal adenocarcinoma does not seem to have prognostic significance, but may reflect developmental differences between the right and left side of the colon. The presence of pS2 staining in adjacent nonneoplastic mucosa and in hyperplastic polyps suggests that the epithelium in these areas is of a regenerative or reactive nature.