1. Seventeen healthy controls and 63 patients with idiopathic calcium stone disease of the urinary tract were investigated for urinary calcium and oxalate excretion and for [14C]oxalate intestinal absorption. 2. Under comparable controlled dietary intake a significant increase in calcium excretion as found in patients with stone disease. Oxalate excretion and [14C]oxalate intestinal absorption were mildly but not significantly increased. When patients with stone disease were subdivided into normocalciuric and hypercalciuric subjects, oxalate excretion and [14C]oxalate absorption were significantly increased in the latter. There was a significant direct relationship between calcium excretion and both oxalate excretion and [14C]oxalate absorption. 3. [14C]Oxalate absorption increased significantly in 22 stone-formers when dietary calcium was changed from normal to low. 4. The kinetics of [14C]oxalate intestinal absorption showed that the main difference between normocalciuric and hypercalciuric subjects occurred within the first 6 h after the oxalate-labelled meal. 5. These results confirm that mild hyperoxaluria is a frequent feature of idiopathic calcium stone disease even when patients and controls are studied under controlled dietary conditions. Our data are consistent with the hypothesis that hyperoxaluria is secondary to calcium hyperabsorption and is upper intestinal in origin.