Numerous reports have claimed that because acidic mucin is absent in benign prostatic glands and is present in some prostatic adenocarcinomas, this stain may be an adjunctive aid in the diagnosis of adenocarcinoma of the prostate. However, adenosis that mimics low-grade adenocarcinoma has not been evaluated to date. We studied 28 foci of adenosis for the presence of high iron diamine-alcian blue (HID-AB). Fifteen foci of adenosis (54%) showed strong staining for HID-AB; staining was diffuse in 11 cases and focal in four cases. An additional two cases (7%) showed equivocal staining. The remaining 11 cases (39%) lacked positivity. All cases of adenosis were verified with immunohistochemistry for keratin 903, a basal cell-specific antibody. This study demonstrates the limited use of acid mucin staining in the diagnosis of adenocarcinoma. The finding of HID-AB positivity in occasional isolated benign small prostatic glands within hyperplastic nodules suggests that acid mucin secretion may be a reflection of gland size or proliferation rather than evidence that adenosis is related to adenocarcinoma of the prostate.