X-linked familial hypophosphatemic rickets (X.L.F.H.R.) is one of the D 
resistant rickets. The inheritance pattern is related to the X chromosome. Most 
constant feature is hypophosphatemia. Pathogenesis is still a subject of 
controversy. There are three main theories: a) An abnormal vitamin D metabolism. 
b) Secondary hyperparathyroidism developping as a result of the diminished 
calcium absorption by gut. c) A primary deffect of phosphate transport al 
various levels. Authors study and comment six cases of X.L.F.H.R., three of 
which belong to the same family. Clinical, radiological and higtological 
findings correspond to those of severe rickets. It is a chronic disease which 
affects children during growth period, giving rise to deforming bones 
invalidism. Treatment consists on continuous administration of oral phosphate 
and vitamin D.