To the Editor.
—Dr Cobleigh and colleagues1 state that the 27 studies they analyzed "do not consistently demonstrate an increased risk of breast cancer among women who have ever used ERT [estrogen replacement therapy]." However, they fail to recognize that most "negative" studies do not have sufficient power to rule out clinically significant differences (see confidence intervals in Table 1 of Cobleigh et al). Since the most reliable estimates of association of any factor with a disease are obtained from prospective cohort studies rather than retrospective case-control studies, it is important that a prospective analysis based on the Nurses' Health Study2 showed a moderate increase in the risk of breast cancer associated with current use of ERT. Bergkvist et al3 also found an increased risk of breast cancer in their prospective study from Sweden, which, in addition, showed no protection from the addition of progestins.Cobleigh et
Atkins CD. Estrogen Replacement Therapy in Breast Cancer Survivors. JAMA. 1995;273(8):619. doi:10.1001/jama.1995.03520320027028