Abstract: Urinary ionized calcium was determined by a calcium activity electrode in 32 normal persons and in 54 patients with calcium-containing renal stones and without urinary tract infection. It was found to be 38 per cent higher in patients with calcium-containing renal stone in comparison to normal persons. However, this was not statistically significant. No consistant change in total or ionized calcium excretion was produced in normal volunteers by the administration of as much as 5 pints of cranberry juice. In patients with renal stones, the urinary ionized calcium was reduced during the cranberry juice ingestion by 50 per cent, which was statistically highly significant.