The main features of X-linked dominant renal hypophosphatemic rickets are illustrated in this study of two patients who presented with rickets, deformities of the lower limbs, and small stature. Hypophosphatemia secondary to a reduction in renal tubular reabsorption of phosphate and a defect of vitamin D hydroxylation are the hallmarks of the disease. The best measure of renal handling of phosphate is to determine the tubular maximum of phosphate transport normalized for glomerular filtration rate (TmPO4/GFR). Determination of the calcium/creatinine ratio of a random urine sample proved to be a good additional parameter to control supplementation of phosphate and calcitriol. Diagnosis of the disease in our two patients enabled us to recognize renal hypophosphatemic rickets in both mothers and in two further ancestors of one patient. Both mothers suffered from early arthrosis.