For many years it has been perceived wisdom that hormone replacement therapy for women with a uterus should include a progestin to prevent the proliferative effects of estrogen on the endometrium and endometrial cancer. But, with the reports from the Women's Health Initiative (WHI) and Million Women Study indicating that such regimens are associated with an increased risk of breast cancer, whereas unopposed estrogen may not increase this risk, or even reduce it, it is pertinent to reassess the merits of adding a progestin. In addition, the suggestion from the WHI that the effects of estrogen and progestins are a 'class effect' are clearly inaccurate, as there is particular evidence from the French E3N cohort studies of differential effects of progestins, with progesterone and dydrogesterone additions showing no increase in risk of breast cancer. The data are presented but an answer to the posed question remains unclear and as usual dependent on the circumstances and views of each individual woman and her medical adviser.