OBJECTIVE: Because small increments in levels of endogenous plasma estradiol are associated with higher postmenopausal bone mineral density, we investigated the safety and effectiveness in preventing bone loss of unopposed, very-low-dose transdermal estradiol for postmenopausal women. METHODS: This was a randomized, placebo-controlled, double-blind trial with 2-year follow-up at 9 United States clinical centers. The study population comprised 417 postmenopausal women, aged 60-80 years, with intact uterus and bone mineral density z scores of -2.0 or higher, who were randomly assigned to receive either unopposed transdermal estradiol at 0.014 mg/d (n = 208) or placebo (n = 209). All participants received calcium and vitamin D supplementation. Lumbar spine and total hip bone mineral density change was measured by dual-energy X-ray absorptiometry; endometrial hyperplasia incidence was assessed by endometrial biopsy. RESULTS: Median plasma estradiol level in the estradiol group increased from 4.8 pg/mL at baseline to 8.5 pg/mL at 1 year (P .001 versus baseline) and to 8.6 pg/mL at 2 years (P .001 versus baseline) and was unchanged in the placebo group. Lumbar spine bone mineral density increased 2.6% in the estradiol group and 0.6% in the placebo group (between-group difference 2.0%, P .001). Mean total hip bone mineral density increased 0.4% in the estradiol group and decreased 0.8% in the placebo group (between-group difference 1.2%, P .001). Osteocalcin levels and bone-specific alkaline phosphatase were lower in the estradiol group than the placebo group (P .001 each). Endometrial hyperplasia developed in 1 woman in the estradiol group but in none of the placebo group (difference in 2-year rates 0.5%, 95% confidence interval 0-7.3%). CONCLUSION: Postmenopausal treatment with low-dose, unopposed estradiol increased bone mineral density and decreased markers of bone turnover without causing endometrial hyperplasia.