The case of a woman, fifty-four years old, with extensive and severe osteomalacia, probably acquired after the age of forty-eight, is reported. The family history revealed a parental consanguineous marriage but no other significant abnormalities. Serum calcium was 9.0 and 10.4 milligrams per 100 milliters; serum inorganic phosphorus, 0.33 and 0.68 milligram per 100 milliliters; serum alkaline phosphatase, 16 Bessy-Lowry units; and serum carbon-dioxide content, 39.8 volumes per cent. Neither amino-aciduria nor glycosuria was found. The main metabolic disturbance was the extremely low tubular reapsorption of phosphorus and the defective intestinal absorption of calcium and phosphorus. Large doses of vitamin D—300,000 decreasing to 200,000 units a day—corrected both the intestinal and the renal disturbances and were given over a period of a year and a half without signs of intoxication. A remarkable clinical and roentgenographic improvement in the patient's bones was obtained. Although the increased urinary parathyroid-hormone-like phosphaturic activity pointed to the coexistence of secondary hyperparathyroidism, the extremely low tubular reabsorption of phosphorus could not be attributed to the secondary hyperparathyroidism alone.