Calcium absorption was determined in 6 patients with chronic renal disease (GFR values 4.8 to 37 ml/min) on a constant diet before and after administration of 5, 6-trans vitamin D3. Administration of 0.5 to 1.0 mg of 5, 6-trans D3 for 8 days did not improve calcium absorption. In two of these same patients (the most uremic) administration of 5 mg of 5, 6-trans D3 for 8 days increased calcium absorption markedly. Two other uremic patients who were studied under the same conditions and received 5 mg of native vitamin D3 for 8 days had no significant increase in calcium absorption. Since 5, 6-trans D3 normalized calcium absorption in patients with far-advanced renal disease, it is suggested that this analogue of vitamin D may be a useful agent in the treatment of hypocalcemia in patients with renal failure.