The expression of tenascin, an extracellular matrix glycoprotein, was studied in three human prostatic carcinoma cell lines by Northern and Western blot analyses and in human prostate tissues by immunohistochemistry and Western blot analysis. All three carcinoma cell lines expressed tenascin mRNA and protein, which were found predominantly in secreted form in culture supernatant. By immunohistochemistry, fetal prostatic tissue showed strong and diffuse tenascin immunoreactivity around developing glands. Normal adult prostatic tissue revealed only focal, scant periglandular and stromal immunoreactivity around acini and ducts. Most cases of hyperplasia and intraepithelial neoplasia showed variable periglandular immunostaining. Tenascin periglandular staining with diffuse stromal extension was noted with all grades of adenocarcinoma; however, the intensity was variable and appeared unrelated to the histologic grade. Metastatic prostatic carcinoma showed strong immunoreactivity in lymph nodes and bone marrow samples, with only weak reactivity of the normal connective tissue framework in both tissues. Western blot analysis of prostatic hyperplasia and carcinoma demonstrated the large and small isoforms of tenascin. These findings suggest a prominent role for tenascin in stromal alterations associated with both benign and malignant prostatic epithelial growth processes.