Thirty-six patients with recurrent calcium oxalate nephrolithiasis were selected from the stone clinic. Fourteen were normocalcemic and had normal daily urinary calcium excretion. Among 22 patients with idiopathic hypercalciuria, 10 received thiazide diuretics for the prevention of new stone formation. Single-voided urine samples were collected at the outpatient clinic and 24-hour urine at the patients' homes. In hypercalciuric patients, irrespective of thiazide diuretic therapy, the mean value of the calcium/creatinine concentration ratios of postprandial single-voided urine specimens had a meaningful correlation with the man value of 24-hour urinary calcium excretion rates. Also in hypercalciuric patients with thiazide diuretics, a negative correlation was observed between the calcium/creatinine concentration ratio and the index for urinary saturation with calcium oxalate of a postprandial single-voided urine sample. Thus, in the hypercalciuric stone formers, 24-hour urinary calcium excretion rates and the degree of urinary saturation with calcium oxalate can be estimated from the calcium/creatinine concentration ratios of single-voided urinary samples.