We calculated person-years from the return date of the questionnaire (in 1996) to the first date of diagnosis of breast or other cancer (except nonmelanoma skin cancer), death, or December 31, 2000. Cox proportional hazards regression was used to estimate relative risks (RRs) and 95% confidence intervals (CIs) for breast cancer, with adjustment for the following potential confounders: age (continuous), race or ethnic group (white, African American, Hispanic, Asian, or other), family history of breast cancer (yes or no), level of education (<12 years of school or high school equivalent, 12 years of school or high school equivalent, post–high school vocational or technical training, some college or college graduate, or postgraduate), age at menarche (<12, 13-14, or ≥15 years), age at menopause (<40, 40-44, 45-49, 50-54, or ≥55 years), age at first birth (<20, 20-24, 25-29, or ≥30 years or nulliparous), parity (0, 1, 2, 3-4, or ≥5 live births), smoking status (never smoker, past smoker with categories of time since quitting of <5 and ≥5 years, or current smoker with categories of <20 and ≥20 cigarettes per day), physical activity (never, 1-3 times per month, 1-2 times per week, 3-4 times per week, or ≥5 times per week), fat intake (quintiles), alcohol consumption (quintiles), oophorectomy (yes or no), height (<155, 155-159, 160-164, 165-169, 170-174, 175-180, or >180 cm), and MHT use (current, past, or never). In weight change analyses, models were adjusted for weight at the beginning of the change interval to account for differences in body size at the beginning of the interval. We also ran models that mutually adjusted for weight change during all intervals. Tests for linear trend were conducted by treating the median values of each exposure category as a single continuous variable in the model. We also applied nonparametric regression using cubic splines18 to examine the association between weight change and breast cancer risk. To explore the effect of patterns of body size change throughout the lifetime on postmenopausal breast cancer risk, we classified participants as overweight or obese (BMI ≥25) or normal weight (BMI <25) with respect to BMI at ages 18, 35, and 50 years and the current age.