The effects of potassium and magnesium supplementation on urinary risk factors for renal stone disease were studied in 61 renal stone patients. The subjects were divided into four groups and supplemented for a period of one month with potassium chloride (KCl, Group 1), potassium sodium citrate (K Na citrate, Group 2), magnesium glycine (Mg glycine, Group 3) and potassium magnesium citrate (K Mg citrate, Group 4) with a daily dose of 42 mEq potassium, 21 mEq magnesium or sodium and 63 mEq citrate, accordingly. The results showed that serum potassium and magnesium of all four groups normalized after the supplementation. Though urinary potassium significantly increased in all three groups supplemented with elemental potassium containing solutions [i.e. KCl (p < 0.001), K Na citrate (p < 0.001) and K Mg citrate (p < 0.001)] only K Na citrate and K Mg citrate, caused a significant increase in urinary pH and citrate but decrease in calcium. Supplementation with Mg glycine in Group 3 although caused a significant increase in urinary magnesium, its effects on urinary pH, citrate and calcium, however, were similar to KCl, in that they caused a significant decrease in urinary pH without any change in urinary citrate or calcium. Supplementation with K Mg citrate in Group 4 seems to have given the best results, as far as lowering stone risk factors in that it caused an increase in urinary pH, potassium and citrate and decreased calcium excretions similar to K Na citrate in Group 2. In addition, K Mg citrate also caused the enrichment of urine with magnesium, another inhibitor of calcium-containing stones. Although the four supplements had no effect on urinary saturation of calcium oxalate salt, their effects on the saturations of brushite (CaHPO4 x 2H2O), octacalcium phosphate (Ca8H2 (PO4)6 x 5H2O) and uric acid were clearly associated with changes in urinary pH. Therefore, in Group 1 and 3, subjects having a decrease in urinary pH, also experienced a significant increase in uric acid saturation. Though the saturation of brushite and octacalcium phosphate in Group 2 and 4 and the sodium acid urate in Group 2 were significantly increased, these urinary risk factors could be overcome, however, by the concomitant increase in urinary citrate. The present results demonstrate that for those stone vulnerable subjects having a high risk of potassium and magnesium depletion, to obtain the best therapeutic results, they should be provided supplementations of both potassium and magnesium together and also in the forms that would result in the delivery of an alkali loading effect.