Abstract
A very large meta-analysis of the Collaborative Group on Hormonal Factors in Breast Cancer has revealed an increasing risk of breast cancer with longer durations of use of hormonal replacement therapy (HRT). This risk increase is restricted to current users of HRT or women who ceased use recently. A 35% increase of the relative risk of breast cancer was found after 5 years of HRT use or longer. After cessation of HRT use the excess breast cancer risk disappeared in 5 years, even after long durations of use. The risk increase was greater for women of normal or lean body weight than for more obese women. Tumours found in women who ever used HRT were diagnosed at a lower clinical stage, which suggests that enhanced screening might be involved. However, the increased breast cancer mortality in HRT users, as found in other studies, rather suggests a biologic mechanism Long-term use of HRT is not yet common in the Netherlands, but an increasing trend is present. Thus, the benefits of long-term use of HRT must be carefully weighted against the risks.