Postmenopausal hormone therapy (HT) is associated with increased risk of breast cancer. The HTs used in Scandinavia is associated with higher risk estimates than those used in most other countries. Mammographic density is one of the strongest risk factors for breast cancer, and possibly an intermediate marker for breast cancer. We decided to examine the relationship between use of different types of HT and mammographic density in Norwegian women. Altogether, 1,007 postmenopausal participants in the governmental mammographic screening program were asked about current and previous HT use. Mammograms were classified according to percent and absolute mammographic density. Overall, current users of HT had on average 3.6% higher mean percent mammographic density when compared with never users (p < 0.001). After adjustment for age at screening, number of children and BMI in a multivariate model, women using the continuous estradiol (E 2 ) plus norethisterone acetate (NETA) combination had a mean percent mammographic density significantly higher than never users (6.1% absolute difference). Those using the continuous E 2 plus NETA combination had an 4.8% (absolute difference) higher mean percent mammographic density after <5 years of use when compared with never users, while the corresponding number for 5 years of use was 7% (p-trend < 0.001). We found similar associations when absolute mammographic density was used as the outcome variable. In summary, our study shows a statistical significant positive dose-response association between current use of the continuous E 2 plus NETA combination and both measures of mammographic density. ' 2006 Wiley-Liss, Inc.Key words: postmenopausal hormone replacement therapy; mammography; breast density Current and recent use of postmenopausal combined estrogen and progestin therapy (EPT) have been shown to increase the risk of breast cancer, both in Randomized Controlled Trials and in observational studies.1-3 The dose and type of the progestin constituent of EPT seems to influence risk of postmenopausal breast cancer more than the estrogen constituent.3 The risk estimates for breast cancer with current EPT use found in recent Scandinavian cohort studies were higher than those found in both the Women's Health Initiative (WHI) Study and the Million Women Study (MWS).1,2,4,5 This may be attributed to the more potent testosterone-derived norethisterone acetate (NETA) progestin used in EPT's in Scandinavia compared to the less potent medroxyprogesterone acetate (MPA) progestin used in most other countries. Mammographic density is one of the strongest independent risk factors for breast cancer, and possibly an intermediate marker for breast cancer.6 Percent mammographic density has consistently been shown to be strongly associated with breast cancer risk in different populations, 6 as well as associated with several breast cancer risk factors. [7][8][9][10] There is substantial evidence that hormones are associated with mammographic density, both from cross-sectional and clinical trials.11-...