BACKGROUND Obesity and high breast density both increase breast cancer risk but paradoxically are inversely related. Bariatric surgery decreases breast cancer risk, but its impact on mammographic breast density is not well understood. OBJECTIVE We investigated how mammographic density changes after bariatric surgery and whether this change is related to weight loss. METHODS University of California, San Francisco Medical Center. METHODS We reviewed records from 349 prospectively collected patients who underwent bariatric surgery between 2013 and 2015 and identified 42 women with pre- and postoperative screening mammograms within 1.5 years of surgery. We recorded body mass index (BMI), height and Breast Imaging Reporting and Data System density and calculated BMI loss and total weight loss. Data were analyzed in Stata 14.2. RESULTS Average age was 54.2 years, mean preoperative BMI was 43.8 kg/m2, mean BMI lost was 30.9%, and total weight loss was 31.1% at 1.3 years. Over one-third had a change in mammographic breast density, which increased 93.3% of the time (P < .001). Amount of weight loss was not associated with a density change. Patients with the lowest mammographic density preoperatively were most likely to have a density change (P = .02). CONCLUSIONS Most women with a mammographic change had an increase in breast density, despite bariatric surgery being associated with reduced breast cancer risk. Baseline breast density was associated with a density change, but amount of weight loss was not. These findings suggest the metabolic effects of bariatric surgery have an effect on breast parenchyma independent of absolute BMI reduction or weight loss.