In the present study it has been found that hyperuricosuric calcium oxalate (CaOx) stone formers do not differ from idiopathic CaOx stone formers in their urines' potential to retard in vitro precipitation of CaOx. On the other hand, urines of hyperuricosuric patients with no history of CaOx stone formation have the same potential to inhibit CaOx precipitation as those of normal controls. Reduction of urinary uric acid concentrations by either incubation of specimens with sodium urate or by treatment of hyperuricosuric patients with allopurinol had no effect on the urines' potential to retard CaOx precipitation.