Background: Although prostatic adenocarcinoma is the most frequent malignancy among American men, little is known concerning the roles of growth factors, growth factor receptors, or proto-oncogene products in the development and progression of this malignancy. Purpose: We examined and compared the expression and cellular distribution of the erbB-3 protein (p160erbB-3) and the erbB-2 protein (p185erbB-2) in various stages of development and progression of prostatic adenocarcinoma. Methods: Immunoperoxidase staining was used to determine the expression of p160erbB-3 and p185erbB-2 in benign prostatic epithelium, prostatic intraepithelial neoplasia, localized adenocarcinomas (pathologic stages B and C) as well as matching primary and lymph node lesions from patients with stage D adenocarcinoma. In order to test antibody specificity, we used Western blot analysis to examine the expression of p160erbB-3 and p185erbB-2 in selected prostatic adenocarcinomas. Results: Within benign glands, immunostaining for p160erbB-3 or p185erbB-2 was strongest in the basal cells and was typically absent or weak in the luminal (secretory) cells. Expression of both proteins within luminal cells was localized to cell membranes. We examined the expression of p160erbB-3 and p185erbB-2 in the prostatic intraepithelial neoplastic lesions of 22 radical prostatectomy specimens. Like benign glands, the basal cells of these neoplastic lesions typically demonstrated strong to moderate immunostaining when stained with either antibody. In contrast to benign glands, moderate to strong immunostaining for p160erbB-3 and p185erbB-2 was frequently observed within the cytoplasm and cell membranes of the prostatic intraepithelial neoplastic luminal cells. p160erbB-3 and p185erbB-2 were detected within the malignant cells in 28 and 29 of 29 localized adenocarcinomas, respectively. Immunostaining with either antibody was typically moderate to strong in the malignant cells. Both proteins were expressed on the cellular membranes as well as in the cytoplasm of malignant cells. Immunostaining for p160erbB-3 was detected in the matching primary and metastatic lesions (lymph nodes) in 10 of 11 patients with stage D adenocarcinoma. Immunostaining for p185erbB-2 was detected in matching primary and metastatic lesions (lymph nodes) obtained from 16 patients with stage D adenocarcinoma. Western blot analysis confirmed the specificity of the antibodies. Conclusions: These results suggest that increased expression and changes in the subcellular distribution of both p160erbB-3 and p185erbB-2 represent early events in the development and progression of prostatic adenocarcinomas. The high expression and distribution of both p160erbB-3 and p185erbB-2 are retained both in advanced-stage primary and metastatic tumors. © 1994 Oxford University Press.