Polyethylene glycol (PEG) solution is currently used to prepare the colon before colonoscopy. Hyponatremic encephalopathy is a condition with neurologic symptoms such as headache, vomiting, confusion, seizure, and sometimes respiratory arrest. A 55-year-old woman presented to our emergency department with generalized tonic-clonic seizure and mental change after ingestion of PEG solution 2 L with water. Serum sodium concentrations was 115 mmol/L. After correction with hypertonic saline for 12hrs, serum sodium concentration reached 129 mmol/L and mental status recovered. We experienced a case of acute hyponatremic encephalopathy resulting from ingestion of PEG solution 2 L for colonoscopy.