Vesicoureteral reflux (VUR) may occur intermittently and cyclic voiding cystourethrography (VCUG) can enhance the ability of the method to detect reflux. We undertook this prospective study to assess how often VUR may occur intermittently during VCUG and to evaluate the reliability of the method by performing cyclic VCUG. Two hundred seventy-five children younger than 2 years underwent two cycles of VCUG. Ninety-seven refluxing kidney-ureter units (KUU) from 68 children were identified during the two cycles. In 18 children VUR was demonstrated in the first, and in 50 children only in the second, cycle. Discrepancy between the two cycles regarding the presence and/or grade of VUR was observed in 85 KUU from 63 of 275 children (23%). In 21 of these 63 children VUR was > or = grade III. In the presence of reflux in the first cycle, discordant findings in the second cycle were found in 11 of 23 KUU (48%) or in 13 of 18 children (72.2%). In the absence of VUR in the first cycle, the second cycle disclosed reflux in 50 of 257 children (19.5%). In conclusion, intermittent VUR occurred in up to 23% of children undergoing VCUG. In more than one-third of them VUR was of major degree. Cyclic VCUG can enhance the ability of the method to detect and grade reflux.