Gonadal dysgenesis is defined as congenital hypogonadism related to abnormalities of the sex chromosomes. Because sex steroids play a central role in the acquisition and maintenance of bone mass, studies have been done to investigate bone status in patients with gonadal dysgenesis, particularly Turner's syndrome and Klinefelter's syndrome, which are the two most common types. The severe estrogen deficiency characteristic of Turner's syndrome (44, X0) is associated with a significant bone mass decrease ascribable to increased bone turnover, as shown by histological studies and assays of bone turnover markers. Estrogen therapy is followed by a significant bone mass gain and a return to normal of bone turnover markers, suggesting that it is the estrogen deficiency rather than the chromosomal abnormality that causes the bone mass deficiency, although abnormalities in the renal metabolism of vitamin D have been reported. Combined therapy with estrogens and growth hormone seems beneficial during the prepubertal period. In Klinefelter's syndrome (47XXY), serum testosterone levels are at the lower end of the normal range and dihydrotestosterone levels are low. Histological studies show depressed osteoblast function and a decrease in 5-alpha-reductase activity responsible for partial tissue resistance to androgens. Assays of bone turnover markers show evidence of increased bone turnover. The bone deficiency is most marked at the femoral neck and seems correlated with serum testosterone and estradiol levels. Androgen therapy has favorable effects on the bone only if it is started before puberty. Recent data suggest that estrogens may contribute to the development of demineralization in KS and that bisphosphonate therapy may be beneficial.