Background: Premature ovarian insufficiency (POI) is a condition in which women under the age of 40 years have absent or irregular menstrual cycles. POI can cause infertility, signs of menopause, osteoporosis, and other symptoms. Hormone replacement therapy (HRT) is a treatment that gives women extra hormones, such as estrogen and progesterone. HRT works well in adult women. Researchers want to find the most effective doses and regimens for adolescents.

Objective: To monitor the effects of HRT on adolescents with POI.

Eligibility: Female adolescents aged 11 to 19 years diagnosed with POI. Healthy volunteers are also needed.

Design: All participants will have clinic visits every 6 months for 2 years. Each visit may last 2 days. Each visit may include: Blood and urine tests. A test of their heart function. A test to measure the stiffness of their blood vessels. Participants will lie flat with a blood pressure cuff on a leg and a meter on the neck while the cuff inflates. A test of their grip strength. Participants will squeeze a handheld device as hard as they can. Two scans to measure bone density. For one, participants will lie on a table while a scanner passes along their body. For the other, participants will sit in a chair and insert their forearm, then their lower leg, into a scanner. A test to measure skin pigmentation. Participants skin will be touched lightly with a device. An optional visual exam of the vagina. Some vaginal fluid may also be collected with a cotton swab/cytobrush. Participants with POI will receive HRT. They will be given estrogen patches and progesterone pills.