Oral oxalate loading using sodium oxalate or a vegetable juice was done to evaluate the intestinal absorption of exogenous oxalate in 30 patients with renal stones and 13 healthy controls. Fifteen calcium oxalate stone formers, 7 non-oxalate stone formers and 10 healthy volunteers were given an oral loading of sodium oxalate (500 mg). Urinary oxalate increased promptly, reaching a peak value within 4 to 8 hours after administration of a synthetic oxalate orally in a fasting state. In calcium oxalate stone formers, the mean increment of urinary oxalate and the bioavailability following oral sodium oxalate load were significantly higher than in the healthy controls and non-oxalate stone formers. Furthermore, intestinal hyperabsorption of oxalate in our criterion was defined in six patients with calcium oxalate stones (40%). On the other hand, eight calcium oxalate stone formers and three healthy controls were given vegetable juice. Urinary oxalate was increased only slightly after the ingestion, and there was no difference between calcium oxalate stone formers and normal controls. These results suggest that a certain hyperoxaluria might be induced by intestinal absorption of exogenous oxalate, and that the hyperabsorption might indicate a possible risk factor for calcium oxalate stone formation.