Currently, large oral doses of phosphate and 1,25(OH)2D (calcitriol) are the standard treatment of patients with familial hypophosphatemic rickets (XLH). While this therapy is effective in healing the rickets, it is often limited by development of complications due to the high dose of medications required to achieve cure. Among them are the development of calcifications in the kidneys and secondary hyperparathyroidism (HPT) which in some patients may cause complications like high blood calcium level, high blood pressure and damage to the kidney. A drug to treat secondary hyperparathyroidism was just developed. In a short term study we found that it might help the treatment of XLH, by allowing the use of lower doses of the both phosphate and calcitriol. In the present study we will learn if indeed the addition of this new medicine (Cinacalcet) to the long-term treatment will allow the use of lower doses of both phosphate and calcitriol and consequently lower the risk of complications.

Inclusion Criteria:

1. Patients with XLH
2. Age 5-21 years old
3. From the Renal and Endocrinology Clinics who have been in a stable clinical, biochemical and radiological condition for the preceding 9 months
4. Who have not received Cinacalcet before

Exclusion Criteria:

1. Treatment with growth hormone
2. Inability to swallow pills.
3. An orthopedic surgery or braces in the past 9 months, or scheduled within 9 months of start.
4. Pregnancy
5. History of seizure disorder
6. Abnormal liver functions (which may change the drug's AUC)