Pseudoinvasion in colorectal adenomas is often difficult to distinguish from invasive carcinoma. Previous studies have indicated that expression of stromelysin-3 (ST-3), one of the metalloproteinase family of enzymes, may be useful for the identification of early invasive carcinoma. The goal of our study was to detect ST-3 expression in colorectal adenomatous polyps to see if it could be helpful for the differential diagnosis of pseudoinvasion vs. true invasion. We studied 25 polypectomy specimens which were divided histologically into 2 groups; the first consisted of 15 adenomas with invasive carcinoma, 8 of these carcinomas were more diffusely infiltrative (pT1), and 7 tended to be expansively invasive. The second group was composed of 10 adenomas with pseudoinvasion. A 35S labelled cDNA probe was used for in situ hybridization (ISH) and a monoclonal antibody (5ST-4A9) for immunohistochemistry (IHC). The distribution of ST-3 expression as detected by IHC and ISH was identical. All diffusely infiltrative carcinoma cases showed ST-3 expression, but only focally in 2 cases with marked lymphocytic infiltration. None of the expansive carcinoma or pseudoinvasion cases showed ST-3 expression. ST-3 expression seems to be an indicator of invasion, but a negative reaction for ST-3 does not rule out an expansive invasive neoplasm or a diffusely infiltrative invasive tumour with a dense lymphocytic reaction.