Exogenous estrogens and breast cancer.

Rates of breast cancer at Group Health Cooperative of Puget Sound, Seattle, Washington, a health maintenance organization, remained stable from 1972-1979 in women aged 30-44 years and in women aged 55-64 years, despite the fact that the frequency of use of estrogen-containing drugs has varied substantially. By contrast, rates of breast cancer in women aged 45-54 years appear to have fallen since 1977 in association with a substantial fall in estrogen use in women of this age group. The results are consistent with an etiologic role for current exogenous estrogen in breast cancer in middle-aged women.