Overexpression of histone deacetylases has been reported in various human malignancies; however, the expression of histone deacetylases in endometrial tissue is not fully understood. In the present study, the expression of histone deacetylase 1, histone deacetylase 2, and Ki-67 was examined immunohistochemically in 30 normal and 66 malignant endometrial tissue samples. The results were expressed as a positivity index and compared with the positivity index for Ki-67 and rates of patient survival. The effect of 2 histone deacetylase inhibitors, trichostatin A and apicidine, on cell proliferation and the expression of cell cycle regulators such as cyclins (D1, E, and A), p21, p27, and p16 were investigated using 6 endometrial carcinoma cell lines. The positivity index for histone deacetylase 1 (79.8 +/- 33.0, mean +/- SD) and histone deacetylase 2 (106.3 +/- 41.9) was higher in endometrial carcinoma than the normal endometrium, with a significant difference for histone deacetylase 2. The positivity index for histone deacetylase 2 was significantly increased in higher-grade carcinomas (positivity index for grade 3, 124.9 +/- 28.4) compared with grade 1 tumors (86.0 +/- 41.0) and was positively correlated with that for Ki-67. In addition, patients with histone deacetylase 2-positive carcinomas had a poor prognosis compared with those with histone deacetylase 2-negative carcinoma (P = .048). Treatment with trichostatin A or apicidine suppressed the proliferation in all cell lines examined, in association with increased expression of p21 and down-regulation of cyclin D1 and cyclin A expression. These results indicated that increased histone deacetylase 2 expression is involved in the acquisition of aggressive behavior by endometrial carcinoma and suggest histone deacetylase inhibitor to be a promising anticancer drug for this carcinoma.