Significant osteoporosis determined by skeleton radiography and bone densitometry was found in 15 patients with cerebrotendinous xanthomatosis (CTX) whose mean age was 31 +/- 11 years. In three CTX patients, bone biopsies confirmed osteoporosis. Nine patients also sustained bone fractures following minimal trauma. Serum 25-hydroxyvitamin D ([25-OHD] 14.6 +/- 6.6 ng/mL v [normal] 30.4 +/- 8.0 ng/mL; P < .001) and 24,25-dihydroxyvitamin D ([24,25(OH)2D] 1.2 +/- 0.4 ng/mL v [normal] 2.7 +/- 0.8 ng/mL; P < .001) levels were low. Serum concentrations of 1,25(OH)2D, calcium, inorganic phosphorus, alkaline phosphatase, parathyroid hormone, and calcitonin were normal. Patients showed classic manifestations of CTX, including dementia, pyramidal and cerebellar insufficiency, peripheral neuropathy, cataracts, and tendon xanthomas associated with elevated serum cholestanol concentrations. These results demonstrate that extensive osteoporosis and increased risk of bone fractures are components of this inherited disease.