Present menopausal hormone replacement therapy regimens were designed for women who experience ovarian failure around age 50. At least 1% of women, however, experience premature ovarian failure before the age of 40, but there have been no studies establishing optimal hormone replacement therapy regimens for these younger women. Available evidence suggests that the usual menopausal hormone replacement therapy is not adequate to protect young women with premature ovarian failure from developing osteoporosis. Women with premature ovarian failure have abnormally low circulating androgen levels, which may contribute to their risk for osteoporosis. We plan to compare physiological estrogen replacement with physiological estrogen plus androgen replacement in young women with premature ovarian failure to determine which regimen is more beneficial for bone mineral density and other metabolic parameters related to the risk for cardiovascular disease. This study employs transdermal estradiol and testosterone "patches" designed to deliver the same amount for each sex steroid normally produced by the ovary in young women in a physiological manner. Women with premature ovarian failure will be randomized to receive either estradiol plus placebo patches or estradiol plus testosterone patches. Both groups will receive cyclic progestin treatment. A group of age-matched women with normal ovarian function will serve as contemporaneous controls for bone density and other study measures. The primary outcome parameter is femoral neck bone mineral density, to be assessed at baseline, and after 1, 2, and 3 years of treatment. Other outcome parameters include lumbar spine bone mineral density, markers of bone turnover, cardiovascular disease risk factors, and cognitive function.