We measured urinary oxalate and glycolate excretion before and during pyridoxine administration (2 to 200 mg per day) in four patients with primary hyperoxaluria. In two patients with type I primary hyperoxaluria, urinary oxalate and glycolate excretion fell markedly in response to a physiologic dose of pyridoxine of 2 mg per day and became completely normal when the dose was increased to 25 mg per day. In the other two patients, who had a different type of primary hyperoxaluria (normal urinary glycolate excretion), there was no response to 2 mg of pyridoxine per day. In one of these patients, doses of 25 and 50 mg per day were also ineffective, but a moderate reduction in oxalate excretion took place with 200 mg per day; in the other patient there was a moderate reduction in oxalate excretion with 25 mg of pyridoxine per day. Our findings suggest that the degree of hyperoxaluria in this disorder may be only slight or moderate if the patient has been ingesting a pyridoxine-rich diet or multivitamin tablets containing small amounts of pyridoxine. Our results also suggest that smaller doses of pyridoxine than those heretofore employed should be tried in patients with primary hyperoxaluria.