A patient became confused and restless after vesical ultrasonic lithotripsy performed with distilled water as the irrigant. Serum sodium decreased to 120 mmol/litre from the pre-operative value of 138 mmol/litre. A cystogram revealed intraperitoneal extravasation of contrast. She recovered promptly after intravenous infusion of normal saline and emergency surgery for repair of the damaged bladder wall. This case illustrates that hyponatraemia and mental symptoms similar to those following transurethral resection syndrome also occur with ultrasonic lithotripsy when distilled water is used as the irrigant.