Urinary excretion of oxalate, calcium and urate has been investigated in 88 patients affected by idiopathic calcium oxalate stone disease and in 20 normal subjects. Of these ions, only oxalate was found significantly higher in stone formers. Defining hyperoxaluria as urinary oxalate excretion greater than 2 SD above normal, 50% of stone-forming people were found to be hyperoxaluric. When stone formers were classified in normo- and hyperoxaluric, the prevalence of hypercalciuria, hyperuricuria, family history of stone disease and recurrencies in stone formation was the same in both groups. It is concluded that hyperoxaluria is a frequent finding in idiopathic calcium oxalate renal stone disease.