Reduced expression of a cyclin-dependent kinase inhibitor, p27, has been reported to be associated with poor prognosis in several human cancers. The aim of this study was to investigate the potential role of p27 in ovarian cancer development and progression. Immunohistochemical expression of p27 was determined using 117 epithelial ovarian tumor tissues and 8 normal ovaries. p27 mRNA expression was examined by semi-quantitative PCR amplification using 26 ovarian cancer samples. Nuclear staining of p27 was commonly observed in the normal ovarian surface epithelium and the epithelial cells of germinal inclusion cysts. Positive p27 staining rates were significantly higher in serous adenomas (p=0.006) and in serous LMP tumors (p=0.013) than that in serous carcinomas (Fisher's exact test). In serous ovarian cancers, positive p27 staining rate was significantly higher in early stage (stage1/2) than that in advanced stage (stage 3/4) diseases (p=0.030, Fisher's exact test). Log-rank testing showed that negative p27 expression significantly correlates with poor survival in serous ovarian cancer patients (p=0.041). Considerable levels of p27 mRNA were detected in all ovarian cancer samples examined. These results suggest that the underexpression of p27 caused by post-translational mechanism may contribute to the development and progression and result in poor prognosis of serous ovarian cancers.