Anecdotal observation of mammographic changes in patients receiving hormonal replacement therapy prompted a retrospective review of a screening population of 1,500 consecutive patients. Thirty patients were identified who underwent mammography both before or after therapy and during therapy. The duration of hormonal replacement therapy ranged from 1 to 72 months (mean, 22.2 months). An increase in fibroglandular tissue was seen on the mammograms of five of the 30 patients (17%) receiving therapy. The mammograms from the remaining 25 patients (83%) showed no change. A proportion of women receiving hormonal replacement therapy will show increased tissue density at mammography. Awareness of this response should temper concern and, in the absence of suggestive anatomic changes, allow for serial, vigilant monitoring of these patients.