1. Five patients with the osteomalacia of chronic renal failure were given 50--100 nmol of 25-hydroxy vitamin D3 intravenously per day for 24--28 days. 2. In all five patients, during administration of 25-hydroxy vitamin D3 there was a substantial rise in the plasma concentration of 25-hydroxy vitamin D from initially abnormally low values. 3. Significant improvement in bone mineralization, intestinal calcium absorption and muscle strength occurred in the three patients with the greatest rise in plasma 25-hydroxy vitamin D.