A prospective observational study that began following more than 50,000 California teachers in 1995 has confirmed reports linking hormone replacement therapy to breast cancer, but suggests obesity may offer some protection. Data from 56,867 women enrolled in the California Teachers Study indicate that women who used estrogen therapy for at least 15 years had a 19% increase in the risk of breast cancer, and women who used combined estrogen-progestin therapy had an 83% increase in breast cancer risk.The...