A patient underwent transurethral resection of the prostate (TURP), which was complicated at the outset of the procedure by an inadvertent puncture wound of the dome of the bladder and the peritoneum. Shortly after resection was initiated, the patient developed shoulder pain and a tensely distended abdomen, although at this time the serum sodium concentration remained normal. Over the next several hours the patient developed significant hyponatremia. The prolonged and gradual time course of this development suggests that fluid and electrolyte shifts occurred via diffusion across the peritoneal membrane. This case illustrates a rare but potentially dangerous complication of TURP that requires recognition prior to initiation of appropriate therapy.