FK506 binding protein 65 (FKBP65) belongs to a group of proteins termed immunophilins that have a high binding affinity to immunosuppressant drugs as FK506 (tacrolimus) and rapamycin (sirolimus). Treatment of female premenopausal women with tacrolimus, which binds to FKBP65, has been reported to be followed by a strongly increased risk of ovarian cysts. We performed the present study to reveal how FKBP65 is expressed in the ovary and in ovarian tumors and to see if this expression might be related to ovarian tumor development, a relationship we have found in colorectal cancer. Biopsies from prospectively collected samples from ovaries and benign, borderline, and invasive ovarian tumors were analyzed for expression of FKBP65 by immunohistochemistry. The expression was compared to survival and several clinicopathological parameters. FKBP65 is strongly expressed in ovarian epithelium and in benign ovarian tumor cells. In the ovary, a positive staining was also found in endothelial cells of blood vessels. In non-invasive and in invasive malignant tumor cells, a decreased staining was observed, which was not correlated to stage, histology, or survival. A significant inversed correlation to expression of p53 was found. The differential expression of FKBP65 indicates a role in ovarian physiology as well as in ovarian tumor development. Our observations and the chromosomal localization of the FKBP65 gene indicate a tumor suppressor function of the FKBP65 protein in ovarian carcinogenesis.