OBJECTIVE To compare the effect of oral estradiol (E2) plus vaginal progesterone (P4) against placebo on endometrial thickness, endometrial biopsy pathology, cervical cytology and total cancer incidence among healthy postmenopausal women. METHODS This study is a sub-analysis of the Early versus Late Intervention Trial with Estradiol (ELITE), a randomized, double-blinded, placebo-controlled trial that previously demonstrated that hormone therapy (HT) was associated with less progression of subclinical atherosclerosis than placebo when therapy was initiated within 6 years after menopause but not when it was initiated 10 or more years after menopause. This sub-analysis included only ELITE participants with an intact uterus, who were randomized to either daily oral micronized 17-beta-E2 1 mg/day with 4% vaginal micronized P4 gel 45 mg/day for 10 days each month or placebo. METHODS Participants were evaluated at baseline and annually during a median follow-up of 4.8 years for endometrial thickness as determined by pelvic transvaginal ultrasound followed by an endometrial biopsy when indicated, and cervical cytology and cancer incidence. RESULTS Over up to 80 months of follow-up, participants randomized to oral E2 plus vaginal P4 had progressive and statistically significant increases in endometrial thickness (p<0.001), underwent more endometrial biopsies and had a higher rate of endometrial hyperplasia on endometrial biopsy compared with the placebo group. Due to the close follow-up of participants in the trial protocol, these abnormal findings were effectively treated. CONCLUSIONS Our results suggest that 10 days of vaginal P4 45 mg/day is insufficient to completely oppose the effect of oral E2 1 mg/day on the endometrium. Further studies are needed to test alternative doses or frequencies of administration of vaginal P4 for adequate endometrial protection from E2 therapy among postmenopausal women. ClinicalTrials.gov registration NCT00114517.