Atypical adenomatous hyperplasia (AAH) of the prostate, also known as adenosis, is characterized by a proliferation of prostatic glands with abnormal architectural patterns, but without significant cytologic atypia. In some cases it may be difficult to distinguish AAH from prostatic carcinoma. Additionally, it is not clear whether AAH is a precursor lesion of prostatic adenocarcinoma. P504S, a protein highly expressed in prostatic adenocarcinoma, has been recently shown to be a marker of prostate cancer. The goal of this study is to examine the expression of P504S in AAH by immunohistochemistry. A total of 80 prostate specimens, including 40 cases of AAH (prostatectomy N = 30, biopsy N = 6, transurethral resection N = 4), 20 cases of prostatic adenocarcinomas, and 20 cases of benign prostatic hyperplasia, were studied. Immunohistochemistry for a prostate cancer marker α-methylacyl-CoA racemase (P504S) and a basal cell-specific marker 34βE12 was performed in all the cases. The 34βE12 stain confirmed the presence of patchy basal cells in all 40 cases of AAH. P504S was undetectable in the majority of AAHs (33 of 40, 82.5%), focally expressed in four of 40 (10.0%), or diffusely positive only in three of 40 (7.5%) cases of AAH. Interestingly, two of seven P504S-positive AAHs were found adjacent to adenocarcinoma. In contrast, all benign prostatic hyperplasias (20 of 20, 100%) were negative for P504S, and all 20 cases of prostatic carcinomas (100%) showed a diffuse P504S staining pattern. These findings suggest that AAH is a heterogenous entity. The biologic significance of P504S expression in a small subset of AAH remains to be determined. Because most cases of AAH are negative for P504S, immunostaining of P504S is also of diagnostic value in distinguishing the majority of AAHs from prostatic adenocarcinoma.