Principal analyses utilized individual participator data from all qualified prospective studies that had investigated information on the type and timing of menopausal hormone therapy (MHT) and brought together the epidemiological evidence, published and unpublished, on breast cancer risk correlated with various types of MHT, and analyzed the appropriate randomized evidence. If these relationships were considerably causal, then for females of average weight in developed countries, 5 years of MHT, beginning at age 50 years, would progress breast cancer incidence at ages 50–69 years by about one in every 50 users of oestrogen plus daily progestagen preparations, one in every 70 users of oestrogen plus intermittent progestagen preparations, and one in every 200 users of oestrogen-only preparations. The similar abundances from 10 years of MHT would be about twofold as prominent.