Cyclic voiding cystourethrography (VCUG) was prospectively evaluated to determine its ability to demonstrate vesicoureteral reflux (VUR) in children whose VCUG results were initially negative. The authors also assessed the effect of change in the patient's position on the detection of VUR. Seventy-seven children younger than 3 years of age, with negative results from a VCUG study performed while they were supine, underwent a second cycle of bladder filling after they were placed prone (group 1). Sixty-five children who were also younger than 3 years of age and had negative results from an initial VCUG examination performed in the usual supine position underwent a second cycle of bladder filling, which was also performed with the patient supine (group 2). VUR occurred in three children (4%) in group 1 and in eight (12%) in group 2. Most children (68.8%) in the two groups combined had grade II reflux. Cyclic VCUG increased detection of VUR, which led to a change in clinical treatment. Prone positioning did not enhance detection of VUR to the same degree as did multiple studies performed with the patient supine.