Serum parathyroid hormone (PTH) levels are effectively decreased by cinacalcet hydrochloride (HCl) in patients with secondary hyperparathyroidism. We assessed the impact of cinacalcet HCl on bone histology in these patients. Four hemodialysis patients with secondary hyperparathyroidism (intact PTH > or = 300 pg/mL) were treated with cinacalcet HCl with low-doses of vitamin D sterols as well as calcium-based phosphate binders for 52 weeks. Patients 1, 2, 3 and 4 were aged 55, 65, 61 and 70 years old, and the duration of hemodialysis in the patients was 84, 176, 125 and 216 months, respectively. Serum intact PTH, serum bone metabolism markers and bone histomorphometric parameters were determined before and after 52 weeks of the treatment. Serum intact PTH decreased from 1110, 880, 330 and 980 pg/mL to 233, 80, 88 and 116 pg/mL, respectively, in the four patients after 52 weeks of treatment with cinacalcet HCl. Serum levels of bone metabolism markers and all of the histomorphometric resorption parameters decreased in these patients. In particular, fibrosis volume decreased to 0% in all of the patients. Static formation parameters, including osteoblast surface and osteoid-related parameters, all decreased after the treatment, indicating an increase of mineralized bone volume during the treatment. Dynamic parameters except for activation frequency decreased after the treatment, indicating significant suppression of bone turnover. Cinacalcet HCl with low-doses of vitamin D sterols suppressed serum PTH with no significant changes of serum calcium levels. In addition, long-term administration of cinacalcet HCl improved hyperparathyroid bone diseases in patients with secondary hyperparathyroidism.