Using a 2-hour <sup>47</sup>Ca absorption test, significant depression of active calcium absorption was demonstrated in 48 vitamin D untreated haemodialysis patients. This malabsorption of calcium could be corrected by the daily oral administration of 1–2 µg of 1αOHD<sub>3</sub> and 1–1.5 µg of 1,25(OH)<sub>2</sub>D<sub>3</sub>. 5 µg daily for 2 weeks of 3-deoxy-1αOHD<sub>3</sub> and 16 and 64 µg daily for 1 week of 24<sup>R</sup>, 25(OH)<sub>2</sub>D<sub>3</sub> proved ineffective. In 32 successfully transplanted patients, restoration of normal or near normal renal function (serum creatinine < 1.9 mg/100 ml) was not always followed by an immediate improvement in active calcium absorption. Calcium absorption, especially in female patients, was adversely affected by the required immunosuppressive prednisone therapy and improvement was slow.