A British study enrolled a large number of postmenopausal women attending a breast cancer screening program and ascertained whether they were or were not taking hormone replacement therapy (HRT) at that time.26 During the followup period, 2,894 users of HRT at baseline developed breast cancer, versus 3,202 never users, producing a relative risk of 1.66. Deaths from breast cancer occurred in 238 users of HRT at baseline and in 191 never users (relative risk = 1.22).

A letter to the editor (Lancet 2003;362:329) argued that the data provided for breast-cancer mortality are somewhat misleading. Compared with never-users of HRT the relative risk of death from breast cancer was raised in current-users. However, this finding should not be interpreted as evidence that HRT increases the risk of mortality among women diagnosed with breast cancer; the overall breast-cancer mortality rate will necessarily be higher in current-users as a result of the higher frequency of breast cancer among such women.

To quote breast-cancer mortality figures for the subgroup of women diagnosed with breast cancer would seem more appropriate—i.e., never-users of HRT, mortality rate 8.2% (238 of 2894) versus current-users of HRT, mortality rate 6.0% (191 of 3202). These figures give a crude relative risk estimate of 0.725 for current-users versus never-users for breast-cancer mortality, indicating a lower risk of death in women taking HRT at the time of their diagnosis with breast cancer than in never-users.