Familiar hypophosphatemic rickets (X-linked hypophosphataemia) occurs very rarely, nevertheless it makes a significant medical problem because of the fact that misdiagnosed or late diagnosed leads to physical development retardation and to severe deformities of extremities. In the case of severe deformities the surgical treatment is needed. We present the case of the 5 year old girl with X-linked hypophosphataemia, who was late diagnosed, in spite of appearance of typical clinical symptoms (short stature and deformities of extremities) and typical laboratory findings (severe hypophosphataemia, serum calcium levels at the lower normal range). This girl had been "observed" (by a family doctor) for 3 years because of "varus deformity of legs". Despite considerably abnormal developmental parameters: height and weight -3 s.d. for age), length of lower limbs >-3 s.d. for age and abnormalities in X-ray images of lower limbs (at the age of 3 5/12 presence of diaphysial deformations and abnormal metaphyses with zones of inadequate mineralization) the girl had not been diagnosed. After the admission to The Department of Pediatrics, Endocrinology and Disease of Adolescents: TRP 69.1%, PO4- 0.7-0.78 mmol/l, Ca 2.24-2.25 mmol/l, Mg 0.78 mmol/l, normal serum level of 25(OH)D (15.4 ng/ml [n. 11-54]), reduced serum level of 1 alpha 25(OH)2D (5.2 pg/ml [n. 15-70]), PTH 56 pg/ml [n. 10-70], ALP 401 U/l, ACP 9.5 U/l. 24-hour urine: PO4- 0.36-0.60 mmol/kg b.w., Ca 0.021 mmol/kg b.w. Probably, the girl will need surgical treatment because of severe bone deformities.