We calculated person-years of observation for each participant from the date of randomization to the date of confirmed cancer, death from any cause, or March 31, 2004, whichever occurred first. We then used Cox proportional hazards regression to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for the development of premenopausal and postmenopausal breast cancer. Analyzed models were adjusted for age and randomized treatment assignment and, in addition, for risk factors for breast cancer assessed at baseline, including body mass index (calculated as weight in kilograms divided by height in meters squared), physical activity, family history of breast cancer in a first-degree relative, history of benign breast disease, age at menarche, parity, age at first birth, multivitamin use, smoking status, alcohol consumption, and total energy intake in premenopausal and postmenopausal women, and age at menopause and postmenopausal hormone therapy in postmenopausal women. When we additionally adjusted, in premenopausal and postmenopausal women, for the presence of a mammogram screening test (yes or no) obtained during the first 12-month follow-up questionnaire, we excluded cases confirmed during the first year of follow-up. For the time-varying analysis of calcium and vitamin D intake, we used Cox proportional hazards regression to calculate HRs and 95% CIs with menopause status updated in 12-, 36-, 60-, and 96-month questionnaires.