The aim of this open prospective randomized study was to evaluate the effect of a 6-month treatment with alendronate on the bone mineral density (BMD) at lumbar spine in patients with central diabetes insipidus. Eighteen patients with central diabetes insipidus and 18 sex- and age-matched healthy subjects entered this study. At study entry, all subjects underwent BMD assessment at the lumbar spine and measurement of serum osteocalcin (OC) and cross-linked N-telopeptides of type I collagen (Ntx). Thereafter, 9 of the 18 patients were randomized to receive treatment with alendronate at a dose of 10 mg, orally, once daily for 6 months (group 1), whereas the remaining 9 patients did not receive any treatment affecting bone status during this period (group 2). After 6 months, bone metabolism and bone density study were repeated in all patients. At baseline, lumbar BMD values (0.86+/-0.03 vs. 1.01+/-0.02 g/cm2; P<0.001) and serum OC levels (4.7+/-0.3 vs. 7.9+/-0.2 microg/L; P<0.001) were significantly lower, whereas urinary Ntx levels were significantly higher [72.0+/-1.9 vs. 64.6+/-1.7 nmol bone collagen equivalents (BCE)/nmol creatinine (Cr); P<0.01] in patients than in controls. After randomization, no difference in lumbar BMD, serum OC, or urinary Ntx was found between patients of group 1 and group 2. At the 6 month follow-up, no difference in serum OC levels was found compared to baseline evaluation in patients of both group 1 and group 2. By contrast, a significant decrease in urinary Ntx levels was found in patients of group 1 (70.3+/-3.0 vs. 75.4+/-2.1 nmol BCE/nmol Cr; P<0.05), but not in patients of group 2 (68.8+/-3.3 vs. 68.5+/-3.0 nmol BCE/nmol Cr; P = NS). A significant increase in lumbar BMD values was found in patients of group 1 (0.88+/-0.04 vs. 0.83+/-0.04 g/cm2; P<0.05), whereas a significant decrease in lumbar BMD values was found in patients of group 2 (0.86+/-0.05 vs. 0.89+/-0.05 g/cm2; P<0.05). Lumbar BMD increased 7.0+/-1.5% in patients of group 1 and decreased 4.2+/-1.8% in patients of group 2 (P<0.001). In conclusion, this study demonstrated that a 6-month treatment with alendronate in patients with central diabetes insipidus was effective in significantly improving BMD at the lumbar spine, which was significantly worsened in untreated patients. Therefore, alendronate treatment could be used in patients with central diabetes insipidus with documented osteopenia or osteoporosis.