β-Catenin plays essential roles in both intercellular adhesion and signal transduction. As a signaling molecule, β-catenin supplies an activating domain to the T-cell factor/lymphoid enhancer-binding factor family of DNA-binding proteins and activates gene transcription. Posttranslational stabilization of β-catenin, leading to elevated protein levels and constitutive gene activation, has been proposed as an important step in oncogenesis. Stabilization of β-catenin can occur through mutation to highly conserved amino acids encoded in exon 3 of the β-catenin gene (CTNNB1). To determine whether this pathway of malignant transformation is important in prostate cancer, we analyzed 104 prostate cancer tissue specimens, 4 prostate cancer cell lines, and 3 prostate tumor xenografts for activating mutations in exon 3 of CTNNB1. Mutations were detected in 5 of the 104 prostate cancer tissue samples. Four of the five mutations involved serine or threonine residues implicated in the degradation of β-catenin. A fifth tumor had a mutation at codon 32, changing a highly conserved aspartic acid to a tyrosine. Mutational analysis of multiple regions from several tumor samples showed that the β-catenin mutations were present focally and therefore may occur during tumor progression.