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Background Yes-associated protein (YAP) in the Hippo signaling pathway is a growth control pathway that regulates cell proliferation and stem cell functions. Abnormal regulation of YAP was reported in human cancers including liver, lung, breast, skin, colon, and ovarian cancer. However, the function of YAP is not known in prostate adenocarcinoma. The purpose of this study was to investigate the role of YAP in tumorigenesis, differentiation, and prognosis of prostate adenocarcinoma. Methods The nuclear and cytoplasmic expression of YAP was examined in 188 cases of prostate adenocarcinoma using immunohistochemistry. YAP expression levels were evaluated in the nucleus and cytoplasm of the prostate adenocarcinoma and the adjacent normal prostate tissue. The presence of immunopositive tumor cells was evaluated and interpreted in comparison with the patients’ clinicopathologic data. Results YAP expression levels were not significantly different between normal epithelial cells and prostate adenocarcinoma. However, YAP expression level was significantly higher in carcinomas with a high Gleason grades (8–10) than in carcinomas with a low Gleason grades (6–7) (p < .01). There was no statistical correlation between YAP expression and stage, age, prostate-specific antigen level, and tumor volume. Biochemical recurrence (BCR)–free survival was significantly lower in patients with high YAP expressing cancers (p = .02). However high YAP expression was not an independent prognostic factor for BCR in the Cox proportional hazards model. Conclusions The results suggested that YAP is not associated with prostate adenocarcinoma development, but it may be associated with the differentiation of the adenocarcinoma. YAP was not associated with BCR.