p14 activates p53 by inhibiting MDM2 expression and arrests the cell cycle in G1 and G2/M. Abnormal p14 expression has been reported in various human cancers. This study investigated p14 expression in endometrioid adenocarcinoma of the uterine corpus in an attempt to clarify its correlation with other cell cycle-regulators and clinicopathologic parameters. The specimen studied consisted of 124 endometrioid adenocarcinomas, 20 normal endometria, and 20 endometrial hyperplasias. Immunohistochemical staining of formalin-fixed and paraffin-embedded tissues was performed using a Catalyzed Signal Amplification System. Cells with >5% positive staining were classified as positive for p14. A staining score of 1 was adopted when the percentage of positive nuclei was <5%, a score of 2 when it was 5 to 50%, and a score of 3 when it was >50%. In normal endometrium, the frequency of positive staining in the proliferative phase and secretory phase was 50% (4/8) and 58.3% (7/12), with staining scores of 1.8+/-0.9 and 1.6+/-0.5, respectively. The frequency of staining in simple hyperplasia (SH), complex hyperplasia (CH), and complex atypical hyperplasia (CAH) was 88.9% (8/9), 25% (1/4), and 42.9% (3/7), respectively; the staining scores were 1.9+/-0.3, 1.3+/-0.5, and 1.4+/-0.5, respectively. Among endometrioid adenocarcinomas, the frequency of staining of well-differentiated (G1), moderately differentiated (G2), and poorly differentiated (G3) adenocarcinomas was 69% (49/71), 64% (16/25), and 42.9% (12/28) respectively, with staining scores of 2.1+/-0.8, 2+/-0.9, and 1.8+/-1, respectively. Thus expression levels of p14 were higher in G1 tumors than in normal endometria or endometrial hyperplasias, and the frequency of its staining in endometrioid carcinomas was inversely correlated with histologic grade. The staining score for endometrioid adenocarcinomas also was inversely correlated with the labeling index (LI) of Ki-67, but not with that of cyclin A, cyclin D1, cyclin E, cdk2, p27, p53, or other clinicopathologic parameters. In conclusion, p14 expression correlated with histologic grade and Ki-67, but not other prognostic factors in endometrioid adenocarcinoma. Long-term follow-up studies are needed to analyze the significance of p14 expression in these tumors.