Objective: To determine the frequency and degree of change in mammographic densities, and new solid or cystic formations in the breast tissue, during different types of hormone replacement therapy (HRT). Subjects and methods: This prospective study included 118 postmenopausal women, 88 under hormone replacement therapy and 30 control subjects. Four types of hormone therapies were compared for their effects on mammograms and sonograms obtained before and during therapy. Mean duration of follow-up was 16.92 +/- 7.65 months in the treated and 21.56 +/- 11.49 months in the control group. Density changes on mammograms were evaluated subjectively. Results: Density increase was recorded in 34% of the patients receiving HRT and in none of the control subjects (P < 0.01). Highest frequency of density increase was found in the groups treated with estrogen plus cyproterone acetate (46%) and with estrogen plus medroxyprogesterone acetate (43%). Frequencies of density increase in the tibolone users, and in estrogen alone users were 28% and 18%, respectively. Degree of density increase was evidently minimal in tibolone users, compared to others. New cysts occurred in six patients receiving HRT (6%)which was not statistically different from the control group (16%) (P > 0.05). New cyst formation was not related to the degree of density increase. New solid mass formation was not observed. Conclusion: Our findings show that mammographic density changes related to HRT are dependent on the selected hormone regimen. Formations of breast cysts or solid lesions do not seem to be related to HRT. (C) 1999 International Federation of Gynecology and Obstetrics.