Calcium stone (renal) formation in patients with hyperuricosuria has been ascribed to the urate-induced crystallization of calcium oxalate. Citrate (0.5mM), added to synthetic medium metastably supersaturated with respect to calcium oxalate, was shown to inhibit heterogeneous nucleation of calcium oxalate by monosodium urate (2 mg/mL). Long-term trial with potassium citrate (60 to 80 mEq/day) was therefore undertaken to determine whether induced hypercitraturia would prevent calcium oxalate stone formation in 19 patients with hyperuricosuria. The treatment produced a sustained rise in urinary pH by 0.55 to 0.85 to the high normal range (6.5 to 7.0). Urinary citrate levels rose by 249 to 402 mg/day to approximate the normal mean value of 643 mg/day. Commensurate with these changes, urinary saturation of calcium oxalate (relative saturation ratio) and the amount of undissociated uric acid declined significantly. However, the urinary uric acid and saturation of monosodium urate remained elevated. Stone formation declined from 1.55 +/- 2.70 per patient-year to 0.38 +/- 1.22 per patient-year during mean treatment period of 2.35 +/- 0.88 years. Stones ceased to form in 16 of 19 patients during treatment. The results provide physicochemical and clinical evidence for the utility of potassium citrate in the management of hyperuricosuric calcium oxalate nephrolithiasis.