In 12 patients with idiopathic calcium oxalate calculi pyridoxine was administered. Within six weeks mean daily oxalic acid excretion decreased from 480 +/- 122 mumol to 336 +/- 83 mumol. Glycolic acid excretion fell from 208 +/- 51 mumol to 153 +/- 26 mumol (normal range: oxalic acid 228-412 mumol/day, glycolic acid 130-290 mumol/day). The reduction of oxalic acid excretion seems to be beneficial in prevention of idiopathic calcium oxalate calculi.