BACKGROUND X-linked hypophosphatemic rickets (XLHR) is a dominant inherited disease caused by isolated renal phosphate wasting and impairment of vitamin D activation. We present a girl with X-linked hypophosphatemic rickets (XLHR) as a consequence of de novo mutation in the PHEX gene. METHODS A 2.2-year-old girl presented with prominent lower limb rachitic deformity, waddling gait and disproportionate short stature (79 cm, < P5; -1,85 SD). On the basis of hypophosphatemia, hyperphosphaturia, high serum level of alkaline phosphatase, normal calcemia, 25(OH)D and PTH, as well as characteristic clinical and X-ray findings, diagnosis of hypophosphatemic rickets (HR) was made. Normal calciuria and absence of other renal tubular disorders indicated HR as a consequence of isolated hyperphosphaturia. The treatment (phosphate 55 mg/kg and calcitriol 35 ng/kg per day), introduced 15 month ago, resulted in a stable normalization of alkaline phosphatase and phosphorus serum levels (with intact calcemia and calciuria), disappearance of X-ray signs of the active rickets and improvement of the child's longitudinal growth (0.6 cm per month). Subsequently, by detection of already known mutation in the PHEX gene: c.1735G>A (p.G579R) (exon 17), XLHR was diagnosed. Analysis of the parental PHEX gene did not show the abnormality, which indicated that the child's XLHR was caused by de novo mutation of this gene. CONCLUSIONS Identification of genetic defects is exceptionally significant for diagnosis and differential diagnosis of hereditary HR.