In the WHI Estrogen-Alone trial, invasive breast cancer incidence did not differ significantly between women randomized to 0.625 mg/d of CEE(Oestrogen) compared with placebo over an average 7.1 years of follow-up. Preliminary results suggesting a lower breast cancer incidence in women in the CEE group were regarded as surprising in relation to prior evidence14 and warranted a more detailed analysis. In the completed trial database, the invasive breast cancer incidence did not differ significantly between the CEE group and the placebo group (HR, 0.80; 95% CI, 0.62-1.04). However, exploratory analyses suggested that CEE might decrease breast cancer incidence in certain subgroups. In contrast, the proportion of mammograms with abnormalities requiring follow-up was significantly increased in the CEE group in the first year and in each year thereafter.