In Reply We read with interest the Keum and Giovannucci Letter to the Editor in response to our report, in which postmenopausal hormone therapy use did not modify the obesity–breast cancer relationship.1 The Women’s Health Initiative (WHI) results differed from several observational studies suggesting that the obesity–breast cancer relationship is only found among never users of hormone therapy.2 As noted in our report, one reason why the WHI results may differ from previous reports may be due to the use of measured height and weight to compute body mass index (BMI) (calculated as weight in kilograms divided by height in meters squared) whereas observational studies use self-reported height and weight, which may be subject to reporting bias.