Hyponatremia is an under-recognized complication of bowel preparation for colonoscopy. It is usually mild and self-limited and rarely severe. To date, only a few reports have been published of marked hyponatremia with severe neurological dysfunction such as loss of consciousness and seizures. Herein, we present a case who had no previous history of seizures or hyponatremia, who developed severe hyponatremia (serum sodium 112 mEq/L) and generalized tonic seizures after receiving polyethylene glycol (PEG) for bowel preparation. The administration of concomitant thiazide was considered to be a major risk factor. We recommend withdrawing thiazide or closely monitoring the sodium level in cases of thiazide user receiving a bowel preparation, even with the relatively safe purgative regimen of PEG.