Localization and immunohistochemical staining patterns of endometrial protein PP14 were studied in 55 patients providing 86 histologically confirmed pelvic and ovarian endometriotic implants. The cellular localization of PP14 in endometriotic implants is comparable with that in the endometrium: epithelial cells express PP14, whereas stromal cells are negative. Positive immunostaining is restricted to apical secretory-like granules of the epithelial cells. In endometriotic implants with positive staining, PP14 is expressed by some, but not all, glandular epithelial cells. Furthermore, positive staining for PP14 is observed most frequently in foci with in-situ secretory differentiation, whereas implants with proliferative or atrophic implants only rarely express PP14. Finally, PP14 can be localized in endometriotic foci at different depths of infiltration, although positive labelling is seen more often in superficial implants. These data demonstrate that PP14 can be expressed by endometriotic glandular epithelial cells with secretory cellular differentiation and that the histological appearance of ectopic implants sometimes only poorly reflects their function.