Wnt signaling is known to be involved in metabolic bone disorders. Serum levels of sclerostin, a bone-specific protein that inhibits Wnt signaling, have been investigated in a variety of metabolic bone disorders. Serum sclerostin levels are positively correlated with bone mineral density in patients with osteoporosis. Elderly women with high serum sclerostin levels, however, are at increased risk of bone fractures. Since serum sclerostin levels are low in primary hyperparathyroidism and high in hypoparathyroidism, parathyroid hormone could be classified as a factor that regulates sclerostin levels. Serum sclerostin levels are high in glucocorticoid-induced osteoporosis and diabetes mellitus, which feature reduced bone formation. Finally, serum sclerostin levels increase with decreasing renal function. These findings highlight the potential of serum sclerostin levels as a new index for bone assessments which are different in nature from bone mineral density and bone metabolic markers.