The expression of the neu oncogene product was investigated immunohistochemically in 36 cases of benign prostatic hyperplasia (BPH) and seven cases of adenocarcinoma of prostate (CaP). c-neu oncogene encodes a transmembrane growth factor receptor that has partial structural homology with EGF receptor, and is overexpressed and amplified in a number of human tumors, specially, breast cancers. Using a monoclonal antibody, AB-3, which recognizes -COOH-terminal of neu oncoprotein, we have analyzed immunohistochemically the expression of this protein in buffered formalin and Zamboni fluid-fixed surgically removed tissues. Focal patchy and/or diffused cytoplasmic staining of varying intensity was observed in 34 of 36 BPH cases. Four cases showed cell membrane staining as well (4/36 = 11%). All seven cases of adenocarcinomas had moderate to strong c-neu immunoreactivity, and two gave a distinct cell membrane-positive reaction (100%). The available data indicate that prostatic tumors as well as a high percentage of prostatic hyperplasia tissues express c-neu protein; however, its role in cellular proliferation needs further study.