The product of the nm23-H1 gene has been reported to be related to the metastatic potential of several tumors. Although several studies have characterized the expression of the nm23-H1 gene product in prostatic adenocarcinoma, little is known of the expression of this protein in prostatic intraepithelial neoplasia (PIN), a putative precancerous lesion. The authors used immunohistochemistry to examine the expression of the nm23-H1 protein in PIN as well as benign and malignant prostatic tissue. A monoclonal antibody to nm23-H1 (Novocastra, Newcastle upon Tyne, UK, clone 37.6) and a biotin/strepavidin detection system were used for antigen localization. Weak to moderate immunostaining was consistently detected in the benign glandular epithelium of 28 radical prostatectomy specimens. In contrast, strong immunostaining was detected in the glandular epithelium in PIN lesions of 19 radical prostatectomy specimens examined. Strong immunostaining was also frequently detected in the malignant cells of 39 localized prostatic adenocarcinomas, as well as 7 metastatic lesions. These findings show a phenotypic similarity of PIN to prostatic adenocarcinoma with respect to nm23-H1 expression. Furthermore, strong expression of nm23-H1 likely represents an early event in the development of prostatic adenocarcinoma.