Prolactin-inducible protein (PIP), also known as gross cystic disease fluid protein 15, is a predominant secretory protein in various body fluids, including saliva, milk and seminal plasma. Immunohistochemistry of this protein has been exploited as a clinical marker for breast cancer and Paget's disease. This study comparatively examined PIP expression in normal prostate tissues and in adenocarcinomas of the prostate. Quantitative real-time RT-PCR revealed low-level presence (6%) of PIP mRNA in normal prostate tissue in comparison with the seminal vesicle. Indirect immunostaining with monoclonal antibody 3E7 displayed a positive sign for benign epithelium in 8 cases (29.6%) among 27 normal specimens; however, the incidence significantly increased to 56.1% (37/66) in instances involving primary prostate carcinoma tissues of different types. Quantitative RT-PCR also demonstrated that PIP transcript levels in carcinoma regions were significantly higher than corresponding levels in benign regions. These findings conclusively showed that benign prostate epithelium expresses PIP at low levels; in contrast, PIP is over-expressed in carcinomas of the prostate.