The effect of calcium supplementation, in patients with osteoporosis is still a matter of controversy. Oyster shell electrolysate (OSE) was reported to raise serum calcium and increase urinary calcium excretion in vitamin D-deficient states more readily than calcium carbonate. Since the effect of calcium salts on osteoporosis depends heavily on its bioavailability, the effect of 900 mg/day calcium as OSE was tested in 12 elderly osteoporotic females, using radial bone mineral density measured by single photon absorptiometry and spinal trabecular bone density measured by quantitative computed tomography (QCT) as indicated, in comparison with 21 untreated controls in the same geriatric hospital. Radial bone mineral density significantly increased from the pre-test value after 12 and 24 months in subjects given OSE by paired t-test, whereas it fell significantly in the controls. The spinal QCT value on OSE did not change significantly in either the subjects under treatment with OSE, or the controls. Thus OSE may favorably influence osteoporosis by providing a readily available source of calcium.