Two siblings (boy and girl) born to a mother with familial hypophosphatemic rickets had abnormal values of serum phosphorus and serum alkaline phosphatase at the age of six weeks. At this age therapy with 1 alpha-hydroxycholecalciferol (1 alpha OHD3) and phosphate was started resulting in both siblings having normal growth of body length and radiological healing of the bone lesions but persistently low values of fasting serum phosphorus during the time of observation up to 60 and 26 months of age, respectively. Phosphate and 1 alpha OHD3 have a positive influence on serum phosphorus through their effect on the intestine. Neither of the patients developed hypercalcaemia during treatment. It seems, therefore, that the early administration of 1 alpha OHD3 with phosphate in infants with familial hypophosphatemic rickets prevents dwarfism and has a positive effect on intestinal absorption of phosphorus but not on fasting hypophosphatemia.