To our knowledge, the reliability of the voiding cystourethrogram for showing vesicoureteral reflux has not been established. Therefore, we evaluated the procedure in 207 children, each of whom underwent voiding cystourethrography with two or more bladder fillings and voidings. The results showed a discrepancy of presence and/or grade of vesicoureteric reflux from one filling to the other in 22 (12%) of 177 patients with two cycles and in six (20%) of 30 patients with three cycles. No changes were observed with cyclic voiding in cases of grade IV reflux (large, tortuous ureters with pelvic dilatation and caliceal clubbing). The grading of reflux was upgraded from 0 to grade I (into ureter only), II (ureter and collecting system), III (distension of pelvis and calices) by a second voiding cycle in only 3% of patients and by a second and third cycle in 4%. Reflux was downgraded by the second or third filling in a similar percentage. Most changes occurred between minor grades of vesicoureteric reflux. With two fillings, the percentage of agreement of the test (including all grades of reflux) for patients with abnormal findings during the first voiding study was 64%; the percentage of agreement for patients with normal results on the first voiding cystourethrogram was 96%. Voiding cystourethrography is a less reliable test for grading reflux than for documenting the absence of vesicoureteric reflux. Although voiding cystourethrography is a good test to rule out reflux, its diagnostic reliability can be enhanced by a second voiding cycle.