Significant decreases in bone mineral density (BMD) occur on the hemiplegic side in chronic stroke patients, which correlate with the degree of paralysis and hypovitaminosis D. In this double-blind, randomized, and prospective study of 98 patients with hemiplegia involving both an upper and lower extremity (55 males and 53 females; mean age, 71.4 ± 0.6 years) after an acute stroke, 49 were given etidronate for 56 weeks and 49 received a placebo. The BMD was measured by computed X-ray densitometry (CXD) of the second metacarpal bone bilaterally. Forty age-matched control subjects were followed for 56 weeks. At baseline, both groups had 25-hydroxyvitamin D [25(OH)D] insufficiency, increased serum ionized calcium and pyridinoline cross-linked carboxy-terminal telopeptide of type I collagen (ICTP), and low serum concentrations of parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D [1,25(OH)2D], suggesting immobilization-induced hypercalcemia and inhibition of renal synthesis of 1,25(OH)2D. The BMD on the hemiplegic side decreased by 2.3% and 4.8% in the etidronate and placebo groups, respectively (p = 0.0003). After treatment, the serum 1,25(OH)2D concentration increased by 62.2% in the etidronate group and decreased by 12.4% in the placebo group. The etidronate group had significant decreases in the serum ionized calcium and ICTP and increases in PTH and bone Gla protein (BGP), whereas the placebo group had higher serum calcium and ICTP concentrations but stable PTH. These results suggest that etidronate can prevent decreases in the BMD in hemiplegic stroke patients because it decreases the serum calcium through inhibition of bone resorption and causes a subsequent increase in the serum 1,25(OH)2D concentration.