A group of 21 patients with hydronephrosis but normal bladders had their upper urinary tract pressures monitored while perfusing the affected kidneys through a fine bore nephrostomy tube, initially with the bladder on free drainage and subsequently during bladder filling. In 7 patients the renal pelvic pressures were altered by bladder filling and loin pain was reproduced in 4. It was concluded that with high upper tract flow rates, vesical filling can affect upper tract dynamics and this should be considered when evaluating patients with loin pain or potential obstructive uropathy.