Prostatic ductal (endometrioid) adenocarcinoma has been considered a distinct pathologic and clinical entity since it was first described more than 30 years ago. Its current status as a unique neoplasm is controversial, however, because it has considerable histologic overlap with typical acinar adenocarcinoma, particularly in small specimens such as needle biopsies. There are also conflicting views regarding its clinical behavior. We recently encountered a series of typical peripheral zone cancers of the prostate gland with prominent papillary or cribriform pattern that apparently did not involve the large periurethral prostatic ducts or verumontanum. To determine the incidence of these "ductal features" in nonductal carcinoma, we reviewed the findings in 338 consecutive totally embedded whole-mount prostatectomy specimens with typical clinical and pathologic features of acinar carcinoma. We defined carcinoma with significant "ductal features" as one that displayed papillary or cribriform pattern involving an arbitrarily defined aggregate focus at least 5 mm in diameter. Anti-keratin 34beta-E12 immunohistochemical staining for basal cells allowed exclusion of areas of papillary or cribriform pattern of high-grade prostatic intraepithelial neoplasia. We identified carcinoma with ductal features (papillary or cribriform growth) in 17 prostatectomy specimens (5% of cases) exclusively in the peripheral zone without involving the periurethral region. Papillary pattern was present in 11 of these cases (65%) and cribriform pattern in 10 (59%), including 4 cases (24%) with both patterns. Of 11 needle biopsy specimens available for examination from these 17 cases, 4 (36%) contained at least focal papillary or cribriform pattern of carcinoma. We conclude that adenocarcinoma arising in the peripheral zone of the prostate gland may display ductal carcinoma features (papillary or cribriform growth) classically associated with ductal adenocarcinoma. These findings, together with the recognized near-constant association of prostatic ductal adenocarcinoma and typical prostate cancer, suggest that ductal adenocarcinoma results from spread of typical prostatic acinar carcinoma into the large accommodating periurethral ducts and stroma, and that there are no unique histologic features other than site of growth. Identification of papillary or cribriform growth of cancer in prostate needle biopsies usually results from peripheral zone adenocarcinoma and not ductal adenocarcinoma.