The expression of the 72-kd type IV collagenase has been implicated as an important factor in determining the invasive potential of malignant tumors. Using immunohistochemistry and nonisotopic in situ hybridization, type IV collagenase expression was assessed in benign and malignant prostatic tissue obtained from 117 surgical and autopsy specimens. Diffuse strong staining for type IV collagenase mRNA and protein was identified in the malignant cells of more than 85% of prostatic adenocarcinomas and the dysplastic cells of high grade prostatic intraepithelial neoplasia. Benign hyperplastic epithelium showed moderate expression in basal cells and mild expression in secretory cells. The qualitative patterns of type IV collagenase expression in prostatic epithelium at the protein and mRNA levels in individual cases were identical. There was no correlation between the level of type IV collagenase expression and either tumor grade or stage. In 10% of adenocarcinomas, focal mild to moderate stromal cell immunoreactivity was present but mRNA was not detectable in the stromal compartment in any case. The enhanced expression of type IV collagenase in dysplastic epithelium and prostatic adenocarcinoma suggests it contributes to the development of the invasive phenotype. The vast majority of the enzyme present in these tumors is synthesized by malignant cells and its production by stromal cells is negligible.