While studies have shown that use of postmenopausal hormone therapy with estrogen and progestogen (EPT) increases mammographic density, aspects of this association remain unclear. We examined whether mammographic density differed by type of hormone therapy (HT) used, dose, duration of use, time since last use, and whether the effects are modified by age and body mass index (BMI). Using a cross-sectional design, we recruited 2,424 postmenopausal women aged 50-69 years participating in the Norwegian Breast Cancer Screening Program. Mammographic density was assessed with a computer-assisted method, and we estimated mean absolute and percent mammographic density through multiple linear regression, and adjusting for possible confounders. Mammographic density was higher among current HT users (percent density: 22.6%; 95% CI: 22.1-23.2%) than among former (17.7%; 17.2-18.2%) or never users (16.3%; 15.7-16.8%). The highest density was seen in current EPT users of high-dose norethisterone acetate (NETA) regimens who had a percent density of 26.2% (24.3-28.1%). Results differed when considering the combined effect of age and BMI. The effect of EPT on mammographic density was modified by age and BMI, with no apparent association among the youngest women (aged 50-55) with the highest BMI (BMI ≥ 26). A higher mammographic density was found in EPT users compared to never HT users, particularly in women using high-dose NETA regimens. Age and BMI modified the association between EPT use and mammographic density.