The present invention relates to method and kits for diagnosing and/or treating prostate cancer. The method and kit relate to the determination and/or modulation of the methylation degree of tumor suppressive genes in biological samples. More particularly, CpG-methylation of 14-3-3.sigma. was found in all primary PCa samples analysed, but not in matching normal prostate epithelial cells or benign prostate hyperplasia (BPH). CpG-methylation was accompanied by a decrease or loss of 14-3-3.sigma. protein expression in primary PCa and PCa cell lines. PCa-precursor lesions, known as prostatic intraepithelial neoplasia (PIN), also displayed decreased levels of 14-3-3.sigma. expression, whereas normal prostate epithelial cells and BPH showed high levels of 14-3-3.sigma. protein expression. The generality of CpG-methylation in PCa suggests that silencing of 14-3-3.sigma. significantly contributes to the formation of PCa. Furthermore, the CpG-methylation of 14-3-3.sigma. in PCa may be exploited for diagnostic purposes.