Breast constitutional density may be altered and increased (acquired density) by hormonal interventions such as hormone replacement therapy (HRT). The effects of endogenous (steroids, prolactin, insulin-like factors...) and exogenous (HRT, levonorgestrel IUD, tibolone, tamoxifen, SERMs) hormones will be reviewed. Continuous combined estrogen-progestin preparations are most likely to cause an increase in density. Estrogen alone and tibolone are less likely to cause an increase in density. The sensitivity and specificity of mammography are decreased, with increased risk of interval carcinoma and rate of short interval follow-up from false positive results. The issue with regards to interruption of the hormonal therapy prior to mammography, and the duration of the interruption, are discussed.