This population-based case-controlled study aimed to determine the effects of specific formulations of estrogen and progesterone (EP) hormone therapy on the risk of breast cancer in menopausal women. The study found that the risk of breast cancer differed depending on the type of progestogen administered. There was no apparent increased risk among EP therapy when the women were being treated with natural micronised progesterone. Tibolone, a synthetic steroid hormone with both estrogenic and progestogenic properties was also involved in this study, and was found to increase the risk of breast cancer compared with any EP therapy. It was also strongly associated with estrogen-receptor and progesterone-receptor positive tumours.