We studied the usefulness of a p63/P504S immunostain "cocktail" in evaluation of prostate biopsy specimens containing atypical acini suspicious for adenocarcinoma (AASA), high-grade prostatic intraepithelial neoplasia (HPIN), and small foci of adenocarcinoma and tested the sensitivity and specificity of the immunostain with tissue microarrays (TMAs) constructed from prostatectomy and lymphadenectomy specimens. We selected 40 cases containing a focus of adenocarcinoma (14 cases), AASA (7 cases), AASA with HPIN (7 cases), HPIN (6 cases), and atypical favor benign (6 cases). After p63/P504S immunostaining, 13 cases (33%) were reclassified: AASA with HPIN to HPIN only in 5 cases (13%), atypical favor benign to benign in 4 cases (10%), AASA to adenocarcinoma in 2 cases (5%), and atypical favor benign to AASA and atypical favor benign to HPIN in 1 case (3%) each. The diagnosis of adenocarcinoma was supported by immunostain in 14 cases. In TMA studies, the p63/P504S immunostain for adenocarcinoma and HPIN had sensitivity values of 97.2% and 86.2%, respectively, and specificity values of 99.7% and 81.6%, respectively. P504S stained 64 (74%) of 87 cores of metastatic cancers, and no p63-positive cells were identified in the metastases. The p63/P504S immunohistochemical stain is a sensitive, specific marker for prostatic adenocarcinoma and HPIN and useful in the evaluation of AASA in biopsy specimens.