Collaborative Group on Hormonal Factors in Breast Cancer – In this individual participant meta-analysis of worldwide epidemiological evidence on the association of breast cancer risk with different types of menopausal hormone therapy (MHT) and their long-term effects, investigators reviewed relevant, randomized evidence on this association. They found that, during prospective follow up, 108,647 postmenopausal women developed breast cancer at the mean age 65 years, and 55,575 had used MHT. If these correlations were primarily causal, then for women of average weight in developed countries, 5 years of MHT, starting at age 50 years, would increase breast cancer incidence at ages 50–69 years by about 1 in every 50 users of estrogen plus daily progestagen preparations; 1 in every 70 users of estrogen plus intermittent progestagen preparations; and 1in every 200 users of estrogen-only preparations. The corresponding excesses from 10 years of MHT would be about twice as great, they concluded.