 Rationale: Previous research suggests that women who have their ovaries removed as part of their treatment for breast cancer may live longer if the ovarian surgery is performed during the luteal phase last 14 days of the menstrual cycle rather than the follicular phase the first 14 days of the menstrual cycle. The current study will evaluate this important research question in premenopausal women with metastatic hormone receptor-positive breast cancer who will be treated with surgery and tamoxifen. 