Weight gain after breast cancer diagnosis is very common, occurring in 50-95% of early stage patients undergoing adjuvant chemotherapy, and has been associated with poorer prognosis. The goals of this study are to examine weight gain and treatment-induced menopause in relation to pre-treatment and treatment-related changes in sex hormone levels, and the role of genetic polymorphisms in sex hormone pathways, including potential interactions with energy balance, psychosocial factors, tumor characteristics, cancer treatment and medication use. We will conduct a prospective longitudinal study of weight gain in 600 early stage breast cancer patients from 35-75 years of age. Eligible subjects, after informed consent, will be assessed with respect to data on hormone levels, menopausal status, anthropometry, diet, physical activity and psychological variables at baseline, 6,12 and 24 months, following them from breast cancer diagnosis, through treatment, and after treatment. Genotype data will be determined. The long-term aim of this study will be to follow these women prospectively to examine cancer recurrence and mortality.