There is controversy over the value of ultrasonic screening for detection of vesicoureteric reflux (VUR) in babies.We scanned 300 newborn babies and identified 53 with a dilated renal pelvis or other minor abnormalities. Eventually, 9 of these were investigated by voiding cystourethrography and 3 (5 kidneys) proved to have VUR exceeding grade II. In 3 babies the reason for cystourethrography was persistent renal pelvis dilation; in 3 it was urinary tract infection; and in 3 it was a new ultrasonic sign observed early in the series-ballooning of the renal pelvis during voiding. Whereas persistent dilation of the renal pelvis was a non-specific indicator (absent in 2 of the 5 affected kidneys and present in 5 of those unaffected), ballooning was consistently seen in all 5 affected renal pelvises. This sign, easily obtained in a baby who has been sleeping, deserves prospective assessment for its value in screening.