Twenty-two children with idiopathic hypercalciuria (IH) as well as their parents and siblings were compared to 29 control children and their parents and siblings. Urinary calcium excretion following calcium deprivation or calcium loading was significantly higher in parents and siblings of the IH children than in the corresponding controls. Significantly higher rates of glomerular filtration and increased urinary excretion of sodium potassium and phosphate were found in all family members in the IH group as compared to controls. Significant positive correlations of the five variables studied (glomerular filtration and urinary excretion of calcium, phosphate, sodium, and potassium) were noted within individuals and also within families, both in the IH and the control groups, notwithstanding the lower mean levels in the latter. Serum concentrations of calcium, phosphate, potassium, and sodium were similar in the IH and controls. The distributions of all urinary variables in both the IH and control groups were unimodal with considerable overlap of the two groups, suggesting that IH may be a single entity, possibly representing the upper end of normality. Our data seem to indicate that IH is more likely to be due to nutritional than to genetic factors, since maintenance of sodium potassium homeostasis in the face of increased excretion necessitates increased ingestion of these electrolytes, while increased sodium ingestion is known to be associated with increased urinary excretion of calcium, potassium, and phosphate.