Forty one elderly patients admitted to hospital for acute illnesses were also found to have subclinical osteomalacia. Immediately before discharge, therefore, all were randomised to receive either vitamin D2 25 micrograms daily, alfacalcidol 0.5 micrograms daily, or placebo. Treatment was given for at least three months, those allocated to placebo then being switched to an active drug. Within the first three months of treatment with either of the active drugs most patients had exhibited a fall to normal in osteoid values. In only four treatment periods was there a mild increase in serum calcium concentration, and in no patient was this accompanied by deterioration in renal function. Any increase in serum creatinine concentration was invariably attributable to the underlying disease for which the patient had been admitted in the first place. Subclinical osteomalacia in the elderly may be corrected by relatively low doses of alfacalcidol (0.5 micrograms daily) or vitamin D2 (25 micrograms daily) given for three months. Such treatment is safe and not accompanied by a serious risk of hypercalcaemia or renal impairment.