The onset of physical signs in infants with hypophosphatemic vitamin D resistant rickets (HDRR) has generally been considered to be at the age of 12 months, but the time of appearance of hypophosphatemia and rachitic signs on radiographs remains unclear. We report a prospective study in three neonates whose mothers were HDRR. At birth, despite a low maternal serum inorganic phosphorus (Pi) level, the serum Pi level was normal together with a negligible renal Pi leak in one neonate. At age 3 months, their serum Pi levels, percentages of tubular reabsorption of Pi, and renal tubular maximal rates of Pi reabsorption in relation to the glomerular filtration rate were low except for one infant. Radiographically, their rickets were not apparent at birth but at age 3 months in all. A premature born infant, born at 28 weeks' gestation weighing 1240 g, was diagnosed as HDRR based on hypophosphatemia due to low renal tubular maximal rate of phosphorus reabsorption in relation to the glomerular filtration rate (TmP/GFR) and normal urine Ca excretion at age 5 months. They were initially treated with 1 alpha-hydroxyvitamin D3 (1 alpha OHD3) and later with 1 alpha OHD3 in combination with Pi, which results in healing of the rickets and a normal increase in height. Thus, early detection and treatment of patients born from mothers with HDRR before physical signs of bow-leg and short stature is possible, but the outcome of early treatment requires further study.