Increased luminal calcium decreases potassium secretion in microperfused rat distal tubule. To determine if such an effect is also present in vivo, we evaluated renal potassium excretion in 49 children with idiopathic hypercalciuria (urinary excretion of Ca = 5.5 +/- 1.3 mg/kg/day) and in 214 age-matched control children (urinary excretion of Ca = 1.9 +/- 0.3 mg/kg/day). In comparison to controls, hypercalciuric children had significantly increased levels of sodium excretion (fractional excretion of Na = 0.7 +/- 0.3 vs. 0.6 +/- 0.3%, respectively; p less than 0.001) and decreased levels of fractional potassium excretion (7.2 +/- 2.9 vs. 9.2 +/- 3.4%, respectively; p less than 0.001) and of the transtubular potassium concentration gradient (4.2 +/- 1.5 vs. 5.9 +/- 1.5, respectively; p less than 0.001). All indices of potassium excretion correlated significantly and inversely with urinary calcium excretion (p less than 0.001). After an oral calcium load, performed in 30 hypercalciuric children, the increased rates of urinary calcium excretion were accompanied by increased rates of urinary sodium excretion and by a significant decrease in the transtubular potassium concentration gradient. These results support the hypothesis that increased luminal calcium concentration also inhibits renal potassium secretion in man.