Objective: To evaluate the short-term efficacy of (L)-2-oxothiaolidine-4-carboxylate (OTZ, which reduces urinary oxalate excretion in normal subjects) in the treatment of primary hyperoxaluria type 1 (PH1) in a phase II study. Patients and methods: Two patients with PH1 received intravenous infusions of OTZ (100 mg/kg body weight for 2 h) given every 8 h for four doses. One patient also received a placebo treatment. Urine samples (24-h collections) were obtained before and during OTZ treatment and assayed for oxalate, citrate, creatinine, sulphate and pH. Daily blood samples were assayed for plasma oxalate and serum creatinine. Results: Urinary oxalate excretion was unaffected by OTZ treatment. Plasma oxalate declined in both individuals with OTZ treatment, but the effect was small. Plasma cysteine was normal in one patient, rising from a mean (SD) of 36 (3.7) μmol/L before treatment to a peak of 141 μmol/L after OTZ, but was not detected in samples from the other patient. The ratio of oxalate to creatinine clearances was high in both patients, with mean values of 3.1 and 3.8. Conclusions: Treatment with OTZ did not lead to clinically significant changes in urinary oxalate excretion. The high clearance of oxalate in these patients suggests a substantial renal secretion of oxalate.