The effectiveness of potassium citrate as a potassium supplement was compared to that of potassium chloride in 13 patients with calcium nephrolithiasis treated with thiazide. Thiazide treatment alone reduced serum potassium, urinary calcium and citrate without affecting urinary pH. Urinary saturation of calcium oxalate and brushite decreased but not as much as the decrement in urinary calcium because of reduced citrate-calcium complexation. Potassium chloride supplementation averted thiazide-induced hypokalemia and hypocitraturia without influencing hypocalciuric action of thiazide or urinary pH. The decline in urinary saturation of calcium salts paralleled the decrement in urinary calcium. Potassium citrate supplementation also kept urinary calcium low and corrected hypokalemia. Moreover, it increased urinary pH and citrate above levels in other phases. Thus, the ability of thiazide to lower the urinary saturation of calcium oxalate was accentuated by potassium citrate supplementation but not by potassium chloride supplementation, probably owing to increased citrate complexation of calcium. Moreover, the potassium citrate therapy was more effective than potassium chloride supplementation in reducing the propensity for the spontaneous precipitation of calcium oxalate in urine. In conclusion, potassium citrate supplementation may be superior to potassium chloride supplementation in patients receiving thiazide in whom stones form.