In three different general health surveys, including more than 17,000 individuals, the prevalence of renal stone disease was over 10% of all males and 3% of all females. Stones are thus far more common in the population than is generally appreciated from hospital statistics. Each year approximately 1% of the entire adult male population experiences renal stones. This figure appears to increase rapidly and is now at least twice as high as only 20 years ago. Stone formers as a group had a raised urinary excretion of calcium, but were not particularly accumulated in the upper part of the normal urinary calcium distribution. This pattern was generally caused by an intestinal hyperabsorption of calcium, apparently not dependent on the action of parathyroid hormone. The prevalence of stone disease was closely related to the urinary calcium output.