OBJECTIVE to evaluate the effect of hormone replacement therapy and tibolone on the breast. STUDY DESIGN prospective, controlled, randomized study. SETTING Outpatient Menopause Clinic of the Second University of Naples. PARTICIPANTS forty four women in spontaneous menopause without any risk factor for breast cancer were randomly allocated to three groups: 15 patients (group A) were treated with transdermal oestrogens 50 microg, 2 patches/week for 3 weeks per month, plus acetate nomegestrolo per os 5 mg/die for 12 days per cycle, 17 patients (group B) were treated with tibolone 2.5 mg/die. Twelve patients not given any medication represented the control group (group C). METHODS at the time of recruitment and after at least 12 months of therapy the patients were subjected to a questionnaire aimed at quantifying the slight, moderate or severe presence of the tension/mastodynia symptoms and to a mammographic test to assess the parenchymal pattern according to a quantitative method: type 1 (less than 25% of mammary gland covered by dense tissue), type 2 (from 25% to 75% of total glandular area covered by dense tissue), type 3 (more than 75% of mammary parenchyma covered by dense tissue). Statistical analysis was carried out by means of Fisher's exact test. RESULTS after at least 12 months of treatment in Group A 5 out of 15 patients (33%) showed a trend of increase in mammographic density not statistically significant (P=0.22) when compared with group B in which one patient showed a swift from type 1 to type 2 and another from type 2 to type 3. The analysis of tension/mastodynia symptoms revealed a significantly difference between the two groups (P=0.02): in group A mastodynia appeared in three previously asymptomatic women and increased in five women, with a total increase in the symptomatology in 8 out of 15 patients (53.3%), in group B only in one case (5%) mastodynia turned from slight to moderate. CONCLUSION in postmenopausal women oestroprogestogenic replacement therapy may be associated with an increase in mammographic density and with the onset or increase in mastodynia. On the contrary tibolone does not seem to affect normostructured mammas and may be considered a first-rate replacement therapy in case of mammas showing particular density or benign mastopathies.