Indirect radionuclide cystography (IRC) is a useful technique for the detection and quantification of vesico-ureteric reflux (VUR). Its principal advantage over micturating contrast cysto-urethrography (MCU) is its ability to demonstrate VUR under physiological conditions. Three children (age range 6-9 years) reported here illustrate some physiological features of micturition readily seen only on IRC. The most important of these was uretero-ureteric reflux which, because of bladder activity, is not usually identifiable on IRC or MCU but is probably more common than previously thought and misinterpreted as VUR. © 1990 The Royal College of Radiologists, 38 Portland Place, London W1N 3DG, UK. All rights reserved.