Breast cancer is a heterogenous disease and hormonal factors are involved. Since national differences exist in the use of postmenopausal hormone therapy (HT) and other risk factors, associations between HT and breast cancer should be studied nationally. In Finland, estrogen-progestin therapy is associated with higher breast cancer risk than estrogen-only therapy. Also tibolone and levonorgestrel releasing intauterine device combined with estrogen are accompanied with an increased risk of breast cancer. Hormone therapy possibly promotes the growth of the existing undetectable cancer.