Because epigenetic alterations are believed to be involved in the repression of tumor suppressor genes and promotion of tumorigenesis in endometrial cancers and ovarian cancers, novel compounds endowed with a histone deacetylase (HDAC) inhibitory activity are an attractive therapeutic approach. Clonogenic assay in soft agar and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays showed that many endometrial and ovarian cancer cell lines were sensitive to the growth inhibitory effect of HDAC inhibitors (HDACIs), although normal endometrial epithelial cells were viable after the treatment with the same doses of HDACIs that induced growth inhibition of endometrial and ovarian cancer cells. Cell cycle analysis indicated that their exposure to HDACIs decreased the proportion of cells in the S-phase and increased the proportion in the G0/G1 phases and/or G2/M phases of the cell cycle. Induction of apoptosis was confirmed by TUNEL assay, annexin V staining of externalized phosphatidylserine, and loss of the transmembrane potential of mitochondria. This induction occurred in concert with altered expression of genes related to cell growth, malignant phenotype, and apoptosis. In nude mice experiments, valproic acid significantly inhibited human endometrial and ovarian tumor growth without toxic side-effects. Although there are few clinical trials on these cancers, some clinical trials showed that HDACIs in well tolerated doses have significant antitumoral activities in another cancers. These results raise the possibility that HDACIs may prove particularly effective in the treatment of endometrial cancers and ovarian cancers.