Stromelysin-3 (ST3) belongs to the family of matrix metalloproteinases, a group of proteolytic enzymes which are believed to play a role in tumor invasion and metastasis. In the present study, we report that the ST3 gene, which was initially identified in invasive breast carcinoma, is expressed in most other invasive human carcinomas, but rarely in sarcomas and other nonepithelial tumors. In carcinomas, both ST3 RNA and protein were specifically detected in fibroblastic cells immediately surrounding the cancer cells. In agreement with this observation, the carcinomas which are known to progress without inducing a prominent tumor stroma are also those which usually do not express the ST3 gene. ST3 gene expression was also observed in noninvasive carcinomas of the breast, uterus cervix and bladder, where the probability of detecting ST3 RNA and protein positively correlated with the known risk of these lesions evolving towards invasion. Taken together, these observations further support the hypothesis that ST3 may contribute to tissue-remodeling processes associated with carcinoma progression, and may represent a new prognostic factor to define populations of aggressive tumors.