Objective: To perform a pilot study of the effects on the breast by low- dose intrauterine progestogen combined with estrogen.Design: A prospective pilot study.Setting: University hospital.Patient(s): Twenty postmenopausal women without any previous breast disorder.Intervention(s): Women were treated with a low- dose intrauterine system releasing 20 μ g/24 hours of levonorgestrel in continuous combination with 2 mg of oral E2 valerate.The effects on mammographic breast density, breast cell proliferation, and hormonal levels were followed for 18 months.Main Outcome Measure(s): Change in mammographic breast density and breast cell proliferation.Correlations with levels of hormones, growth factors, and binding proteins.Result(s): Three women showed an apparent increase in density.For the remaining 17 women the changes were only a few percent.Digitized assessment of density showed strong correlations with visual classification scales (rs = 0.96- 0.97). There was no increase in proliferation as expressed by the percentage of MIB- 1- positive breast cells in fine- needle aspiration biopsies.Increase in breast density displayed a positive correlation with patients age (rs = 0.52) and an inverse relationship with levels of E2 (rs = - 0.50) and free T (rs = - 0.50).Conclusion(s): Low- dose intrauterine administration progestogen may develop into an attractive alternative for hormonal therapy in postmenopausal women as endometrial protection may be achieved at very low systemic levels. Objective: To perform a pilot study of the effects on the breast by low-dose intrauterine progestogen combined with estrogen. Design: A prospective pilot study. Letting: University hospital. Patient (s): Twenty postmenopausal women without any previous breast disorder. Intervention (s): Women were treated with a low-dose intrauterine system releasing 20 μg / 24 hours of levonorgestrel in continuous combination with 2 mg of oral E2 valerate. The effects on mammographic breast density, breast cell proliferation, and hormonal levels were For 18 months. Main Outcome Measure (s): Change in mammographic breast density and breast cell proliferation. Corlations with levels of hormones, growth factors, and binding proteins. Result (s): Three women showed an apparent increase in density. For the remaining 17 women the changes were only a few percent. Digitized assessment of density showed strong correlations with visual classification scales (rs = 0.96- 0.97). There was no increase in proliferati on as expressed by the percentage of MIB- 1- positive breast cells in fine-needle aspiration biopsies. Crease in breast density displayed a positive correlation with patients age (rs = 0.52) and an inverse relationship with levels of E2 (rs = -0.50 ) and free T (rs = - 0.50) .Conclusion (s): Low-dose intrauterine administration progestogen may develop into an attractive alternative for hormonal therapy in postmenopausal women as endometrial protection may be achieved at very low systemic levels.