Calcium and oxalate were studied in daily, fasting and postprandial urine specimens from healthy subjects and patients with idiopathic renal calcium stones in response to a test meal free of oxalate, and supplemented with calcium and 14carbon-oxalic acid. The data showed that the amount of oxalate in fasting urine of patients with stones did not differ from that in controls. Generally, patients with stones had considerable postprandial hyperoxaluria in terms of excretion and concentration, associated with a significantly higher degree of supersaturation with regard to calcium oxalate compared to controls. These findings were paralleled by decreased intestinal absorption of 14carbon-oxalate and by unchanged 24-hour urinary oxalate. Although the source of increased postprandial oxalate in patients with stones is not clear the possibility of enhanced de novo synthesis from oxalate precursors is discussed. In patients with different types of calciuria the 2 main risk factors (hyperoxaluria and hypercalciuria) for the process of stone formation are recognizable more readily in the postprandial urine specimens than in fasting or daily urine specimens.