The Sertoli cell tumor (SCT) of the testis is a sex cord stromal tumor, usually sporadic, rarely associated with genetic syndromes. Much remains unclear about the molecular genetic changes involved in SCT and its histogenesis. Recently, nuclear β-catenin immunostaining has been reported in a case of bilateral SCT, but the molecular basis of the aberrant nuclear β-catenin expression remains uncertain. In the present study, β-catenin immunohistochemical assay and mutational analysis of exon 3 of the CTNNB1 gene by direct sequencing were performed in 14 SCTs, 2 of which had an unfavorable clinical course. Immunohistochemical study showed that β-catenin was located in the cytoplasm of tumor cells in 4 cases (28.6%) and in both the nuclei and the cytoplasm in the remaining 10 cases (71.4%). β-Catenin mutations were detected in 10 of the 14 patients (71.4%) under evaluation. Ten of 10 mutation-carrying cases showed strong nuclear and diffuse cytoplasmic β-catenin immunoreactivity. Seven of the 8 CTNNB1-mutated tumors tested for cyclin D1 displayed diffuse immunoreactivity in the nuclei of tumor cells. We conclude that CTNNB1 exon 3 mutations are likely to be involved in the pathogenesis of male SCT with nuclear accumulation of β-catenin and affect the expression of cyclin D1.