In an attempt to evaluate the suggestion that calcium deficiency may produce osteoporosis, 166 patients with osteoporosis of unknown cause were studied. The estimated calcium consumption was significantly lower in the osteoporotic patients than in a group of control subjects matched according to age and sex. Steatorrhea was present in only nine patients and five of these had a history of previous surgical procedures which may have caused the steatorrhea. Sixteen of the thirty-five patients tested had subnormal intestinal absorption of 47Ca when the data were corrected for dietary intakes of calcium. Probable hypercalciuria was found in 18 per cent of the total group. It could not be determined from the data whether the subnormal absorption of 47Ca and the hypercalciuria observed in some patients were the cause or the consequence of the increased bone resorption believed to characterize osteoporosis. However, repeat studies of 47Ca absorption made in ten patients, seven of whom had appreciably increased their dietary intake of calcium in the interim, suggested that the supplementary calcium could be partially absorbed but did not establish that the absorbed calcium reflects skeletal utilization.