Cathepsin D (CathD), a lysosomal aspartyl protease secreted by normal and malignant cells, is considered to be involved in breakdown of the extracellular matrix. Aim of the present study was to determine the frequency and tissue distribution of CathD in normal, hyperplastic and malignant endometrium. Paraffin-fixed endometrial tissue was obtained from premenopausal women in the proliferative phase (n = 5), early secretory phase (n = 4) and late secretory phase (n = 4) as well as glandular-cystic hyperplasia (n = 5), endometrial polyps (n = 5), endometrial polyps from the use of tamoxifen (n = 5), adenomatous hyperplasia (AH) grade I (n = 5), grade II (n = 4), grade III (n = 5) and endometroid adenocarcinoma (n = 5). CathD expression was evaluated with the IRS score and ANOVA analysis was used for statistical evaluation. CathD was primarily localised in luminal and glandular epihelium with little staining in stromal cells. The expression of CathD was significantly higher during the late secretory phase than in the proliferative phase. Highest expression of CathD was observed in the late secretory phase and in glandular-cystic hyperplasia, whereas endometroid carcinoma showed no expression. A continuous increase in CathD expression was observed in AH, with a significant difference between AH grade I and III. In conclusion, CathD was found to be expressed in normal and hyperplastic endometrial tissue. CathD immunostaining in normal endometrial glands varied on the basis of the phase of the menstrual cycle, suggesting physiological functions of CathD in endometrial maturation and degradation. Adenocarcinomas did express significant lower amounts of CathD. Therefore, the prognostic value of this parameter remains uncertain. A continuous increase in CathD immunostaining was observed in AH. Since AH grade III can be considered as a precursor of endometrial cancer, CathD could be a possible parameter for assessing malignant transformation.