BACKGROUND After successful kidney transplantation, hyperparathyroidism can persist in 10%-50% of patients and can harmfully affect bone metabolism. Calcimimetic cinacalcet is a new treatment option in the management of persistent hyperparathyroidism in these patients. METHODS This prospective, clinical study of 11 patients included those who had a serum intact parathyroid hormone (iPTH) concentration >65 ng/L, a serum creatinine concentration was <200 μmol/L, stable kidney graft function, and were >1 year since transplantation. Patients were not treated with drugs other than calcitriol that could influence bone metabolism. During the 6-month observation period, in which the stability of measured parameters was determined, and in the 12-month treatment period (cinacalcet 30 mg/d), we followed serum concentrations of calcium, phosphate, iPTH, creatinine, vitamin 25OH D(3), bone-specific alkaline phosphatase (ALP), osteocalcin, collagen degradation fragments (CTX), urinary calcium excretion, and bone mineral density (BMD). RESULTS During the treatment period, the serum calcium concentration decreased significantly (from 2.50 ± 0.12 to 2.32 ± 0.12 mmol/L; P < .01). Serum iPTH concentration decreased significantly (from 247 [range, 199-362] at time 0 to 198 [range, 165-233] ng/L after 1 month of treatment; P < .05), but increased slightly thereafter. After 6 months of treatment, the serum concentration of ALP and CTX increased significantly, but decreased thereafter. There were no significant changes in the other parameters assessed. Renal function remained stable during the treatment period. The BMD of the lumbar spine, hip, and forearm did not change during the 12 months of treatment. CONCLUSIONS Cinacalcet was effective in treating posttransplant hyperparathyroidism, resulting in decreased calcemia and transient decreased iPTH. ALP and CTX transiently increased during therapy, but other markers of bone metabolism remained unchanged. Twelve months of cinacalcet treatment did not result in a change in BMD. Cinacalcet seems to be a safe drug with no negative effect on renal function.