Primary prostatic duct adenocarcinoma, initially labeled as endometrioid carcinoma of the prostate, is a rare neoplasm that displays exophytic growth into the prostatic urethra with involvement of prostatic ducts. Because this tumour arises from preexisting epithelia, there is a possibility that a remnant basal epithelium may be seen in association with these tumours. If this hypothesis is correct, then prostatic duct adenocarcinoma may possibly be mistaken for high-grade prostatic intraepithelial neoplasia (PIN) on needle biopsies. The distribution of basal cells in this tumour has not been described previously. Nine cases of prostatic duct adenocarcinoma and prostatic adenocarcinoma with focal ductal differentiation were studied immunohistochemically with antibodies specifying cytokeratin 34 beta E12, prostate-specific antigen (PSA), and prostatic acid phosphatase (PAP). All cases were positive for PSA and PAP. In some areas of the tumour in eight cases there was a continuous and discontinuous layer of basal cells surrounding islands of carcinoma. This was found with cribriform, comedo, solid, and papillary components of ductal type adenocarcinoma. It is necessary to be aware of the presence of basal cells in association with primary prostatic duct adenocarcinoma. Differentiation of high-grade PIN from this lesion should depend on complex architectural characteristics and Cytologic features rather than presence of a basal cell layer. This finding confirms that the solid, cribriform, papillary, and comedo components initially grow intraluminally within ducts before invasion into surrounding stroma occurs.