Our objective was to compare serum level of placental protein 14 (PP14) with histological findings in endometrial evaluation of postmenopausal women using hormone replacement therapy (HRT). In a subset of 109 out of 140 women included in a randomized comparative study, serum levels of PP14 were determined after 12 months of use of (1) no HRT; (2) oral micronized 17 beta-estradiol/oral sequential dydrogesterone; (3) transdermal 17 beta-estradiol/oral sequential dydrogesterone; or (4) oral tibolone. Subjects underwent Pipelle biopsy after 12 months. The serum level of PP14 was determined by sandwich enzyme immunoassay (ELISA). The two-tailed t-test and one-way ANOVA or their non-parametric equivalents were used to test for statistical significance. All three HRT regimens were safe with respect to the endometrium. Hyperplastic or malignant changes were not observed. There was a significant difference in the mean values of PP14 between the groups of inactive/atrophic and secretory endometrium (p < 0.01). However, there was a wide range of individual values for PP14 within the groups and a wide overlap in values between the groups of non- substituted and hysterectomized women. The quantitative determination of PP14 in the serum did not provide supplementary information on the substituted endometrium. From this study it can be concluded that the serum PP14 determination is not useful to predict endometrial status under HRT. The relatively high levels of PP14 in hysterectomized patients suggest ectopic production.