Familial hypophosphataemic rickets (XLH) is an X-linked dominant disorder resulting in hypophosphataemia, abnormal regulation of 25-hydroxy vitamin D metabolism, elevated activity of alkaline phosphatase, bone deformities and short stature. In 1995-97 the sequence of PEX gene responsible for the disease was established. The PEX gene spreads 24.3 kb and includes 22 small exons coding a protein belonging to a neutral endopeptidase family. Function of the protein is not known yet. Mutation analysis in patients from North America, Africa and Europe (including Poland) revealed the presence of many different types of the PEX gene mutations. Identified deletions, insertions and substitution are supposed to change the structure of the PEX protein. Active form of vitamin D3, 1-alpha-hydroxylase and phosphate supplementation are now the recommended treatment of XLH patients. Further research is necessary to understand the role of the PEX protein in the pathogenesis of hypophosphatamic rickets.