We examined the expression of two groups of matrix metalloproteinases (MMPs), stromelysin and interstitial collagenase, in human skin cancer by northern blot analysis and in situ hybridization. Stromelysin-3 (ST-3) mRNA was overexpressed more than tenfold in 17 of 19 (89%) specimens of basal cell carcinoma (BCC) but in only three of 13 (23%) cutaneous squamous cell carcinomas (SCCs). Stromelysin-1 and -2 (ST-1/2) mRNA was overexpressed in three of 19 (16%) BCC and three of 13 (23%) SCC. Collagenase mRNA was overexpressed in nine of 19 (47%) BCC and three of 13 (23%) SCC. No mRNA for ST-3, ST-1/2, or collagenase was detected by northern analysis in 21 specimens of adjacent normal skin. Because of these findings, we examined the specific location of the ST-3 mRNA in BCC specimens by in situ hybridization. ST-3 mRNA was particularly abundant in the characteristic stroma adjacent to the invasive basaloid tumor islands of the BCC and absent in the malignant cells. Moreover, ST-3 mRNA was expressed and induced by phorbol ester treatment in adult dermal fibroblasts but not in keratinocytes. In vitro studies have shown that MMPs are involved in the degradation of extracellular matrix molecules. Our finding of ST-3 mRNA overexpression in 17 of 19 (89%) BCC specimens is consistent with a role for this molecule in local invasion of stroma by BCC. Our in situ hybridization data suggested that while ST-3 is not expressed by malignant basal cells themselves, these tumor cells may induce the expression of ST-3 in adjacent nonmalignant stromal elements such as fibroblasts.