Urinary volume in 24-hour urine collections was examined in 50 children with hypercalciuria and urolithiasis or hematuria, 12 with idiopathic calcium oxalate urolithiasis and 36 healthy children. Urinary volume was 22.2 +/- 2.0 ml. per kg. per day in healthy children and 25.4 +/- 2.0 ml. per kg. per day in children with hypercalciuria, and it was similar in children with absorptive and renal hypercalciuria, and significantly lower in children with idiopathic calcium oxalate urolithiasis (12.2 +/- 1.4 ml. per kg. per day, p less than 0.001 from controls and children with hypercalciuria). Volume was not statistically different in hypercalciuric children with and without urolithiasis. Urinary sodium excretion in children with idiopathic calculi was not statistically different from controls. Urine osmolality was similar among the groups. Urinary volume represents a risk factor in children with idiopathic calcium oxalate urolithiasis, and increased fluid intake should be emphasized in such patients.