To the Editors: In their recent article, Boyd et al. (1) sought to examine whether the increased risk of breast cancer associated with postmenopausal hormone therapy was mediated through mammographic density (percentage mammographic density) in three screening programs. They did this by comparing the relative risk of breast cancer associated with hormone therapy with and without adjustment for mammographic density measured at the time of entry to the screening programs when the use of hormones was also recorded. They found that the adjustment for mammographic density made little difference to the relative risk estimates. On this basis, they concluded “that the effects of hormone therapy on mammographic density, and on breast cancer risk, are separate and not related causally”.

In our opinion, this conclusion is not warranted. The question is not whether the increased risk of breast cancer associated with hormone therapy is mediated through mammographic density but...

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