About 80% of all breast cancers are oestrogen sensitive. Patients operated for breast cancer treated with tamoxifen or aromase inhibitors (AI) have a lower incidence of contralateral breast cancer. Based on this knowledge, prophylactic studies comparing tamoxifen with placebo have found a 30% reduction in breast cancer. Unacceptably many women developed endometrium cancer or thromboembolic side-effects in the tamoxifen group. Prophylactic studies have been initiated in recent years comparing AI with placebo. Much is expected from these studies where an 80% reduction in breast cancer incidence is anticipated. Unfortunately, this treatment will have no effect in the 20% of cancers that are oestrogen receptor negative. Chemoprevention is not recommended in Denmark. Some studies have shown that many cases are caused by lifestyle factors. It is thought that the incidence of breast cancer could be reduced by as much as 30% by increased exercise, reduction of alcohol use and avoiding HRT.