Only about 20% of renal stone cases have an unquestionable cause such as hyperparathyroidism, renal tubular acidosis etc. explaining their stone formation. About 20-40% are believed to result from idiopathic hypercalciuria. The purpose of the present investigation was to study the renal excretion of calcium, magnesium, sodium and phosphate in 47 consecutive men with recurring renal stone formation without a demonstrable underlining metabolic disease and, for comparison, 43 normal men. The results are related to previous hypotheses on renal stone formation. No difference in urinary calcium (either concentration or excretion) per day is found between the two groups. Consequently the concept of idiopathic hypercalciuria is questioned. The Mg/Ca ratio in urine is found lower in the stone patients than in the controls, suggesting that the Mg/Ca ratio might be of importance in stone formation.