In an attempt to document the efficacy of potassium citrate in stone formation, 57 patients with active lithiasis (2 or more stones during the preceding 2 years) and hypocitraturia were randomly allocated into 2 groups, with 1 group taking 30 to 60 mEq. potassium citrate daily in wax matrix tablet formation and the other group receiving placebo. In 18 patients receiving potassium citrate for 3 years stone formation significantly declined after treatment from 1.2 +/- 0.6 to 0.1 +/- 0.2 per patient year (p < 0.0001), in 13 patients (72%) the disease was in remission and all patients showed a reduced stone formation rate individually. In contrast, 20 patients taking placebo medication for 3 years showed no significant change in stone formation rate (1.1 +/- 0.4 to 1.1 +/- 0.3 per patient year) and in only 4 patients (20%) was the disease in remission. The stone formation rate during potassium citrate treatment was significantly lower than during the placebo treatment (0.1 +/- 0.2 versus 1.1 +/- 0.3 per patient year, p < 0.001). Potassium citrate therapy caused a significant increase in urinary citrate, pH and potassium, whereas placebo did not. Adverse reactions to potassium citrate were mild causing only 2 patients in the potassium citrate group and 1 in the placebo group to withdraw from the study. In summary, our randomized trial showed the efficacy of potassium citrate in preventing new stone formation in idiopathic hypocitraturic calcium nephrolithiasis.