We studied 101 prostatic adenocarcinomas (72 acinar, 29 ductal) and 16 cases with high-grade prostatic intraepithelial neoplasia (HPIN) immunohistochemically for the expression of β-catenin and compared the staining patterns with those of nonneoplastic prostatic epithelium and 24 colorectal adenocarcinomas. While nuclear staining for β-catenin was evident in 20 (83%) colorectal adenocarcinomas, predominantly membranous staining was observed in 89 prostatic adenocarcinomas (88%); the remaining 12 cases showed no immunoreactivity. In prostatic tumors expressing β-catenin, staining intensity was comparable, increased, and decreased in 81, 4, and 4 cases, respectively, compared with adjacent nonneoplastic prostatic epithelium. No prostatic adenocarcinomas demonstrated nuclear staining. The β-catenin staining characteristics in HPIN were comparable to those in nonneoplastic prostatic epithelium. Negative staining for cytokeratins (CKs) 7 and 20, high-molecular-weight (HMW) CK, and villin and positive staining for prostate-specific antigen (PSA) were seen in 22 prostatic adenocarcinomas examined, in contrast with colorectal adenocarcinomas, which stained positively for CK20 and villin and negatively for CK7, HMWCK, and PSA. These data suggest that the β-catenin signaling pathway acts differently in prostatic than in colorectal tumorigenesis. The immunophenotypes documented herein also might aid in the distinction between prostatic and colorectal origins when metastasis is encountered.