The adolescent and young adult years are a critical window in time for bone mineral accrual. More than 90% of peak bone mass is achieved by 18 years, and data indicate that insults sustained during adolescence and young adulthood may result in permanent deficits in bone accrual. Adult athletes with amenorrhea (AA) have low bone mineral density (BMD) secondary to hypogonadism, associated with increased fracture risk and associated co-morbidities. We will examine whether estrogen replacement will increase BMD and improve measures of bone microarchitecture in adolescents and young women with AA, thus optimizing peak bone mass.