Inclusion Criteria:

• Female
• Perimenopausal as defined by the Stages of Reproductive Aging Workshop (STRAW) criteria, specifically in either of the two following stages: a) early transition defined as changes in cycle length of seven days or more in either direction in consecutive cycles or b) late transition defined as > 60 days amenorrhea and FSH > 40 IU/mL
• Intact uterus/ovaries (i.e. no surgical menopause)
• > 7 self-reported hot flashes per week
• Estrogen therapy not contraindicated
• Able to give informed consent
• Age between 40 and 65 years
• English as first and primary language

Exclusion Criteria:

• Positive pregnancy test or breastfeeding (pregnancy tests will be given to all women)
• Obesity > 33 BMI
• Previous history of endometrial hyperplasia/neoplasia
• Previous history of cancers of the breast or reproductive tract
• History of presence of myocardial infarction (MI) or stroke
• Current clinical diagnosis or a diagnosis within the past year of an anxiety disorder, severe recurrent depression, or severe psychiatric disturbance
• History of head injury with more than 60 minutes loss of consciousness
• History of neurological condition affecting cognitive function (e.g., brain tumor, multiple sclerosis)
• History of developmental disability affecting cognitive function (e.g., mental retardation, attention deficit)
• Current use of CNS-acting medication (e.g., antidepressants, anxiolytics, diphenhydramine)
• History or presence of cerebrovascular accident, sickle cell anemia
• History of alcohol or drug abuse as defined by DSM criteria
• Abnormal vaginal bleeding of undetermined cause
• Untreated or uncontrolled hypertension defined as systolic blood pressure greater than 165 mm hg or diastolic blood pressure greater than 95 mm hg
• Concurrent administration of medication containing estrogen, progestin, SERM within four months of enrollment
• Concurrent administration of medication containing St. John's wort, bisphosphonates, or dietary phytoestrogens within one month of enrollment
• History of migraine associated with hormone use
• History or presence of deep vein thrombosis, thrombophlebitis or thromboembolic disorder
• Current participation in any other clinical trial within 30 days of enrollment
• Smoker
• Diabetes
• Premature ovarian failure (defined as having last menstrual period before age 40)
• Abnormal PAP smear in previous year
• Abnormal mammogram in previous year
• Vegans (vegetarians who tend to consume greater than average doses of phytoestrogens)
• Allergy to soy (affects ~1% of people in the United States; reactions are typically mild)
• Symptomatic fibroids (significant size or significant menstrual changes)
• Menorrhagia
• Lactose intolerant