A 76-year-old woman was referred to our hospital for unresponsiveness and hypotension. She had developed constipation that had led to ileus and had received 34 g of magnesium citrate (Magcolol P^®) orally the day before. She was lethargic, her blood pressure was less than 50 mmHg, and electrocardiogram (ECG) revealed sinus arrest with junctional escape rhythm. Her serum concentration of magnesium (Mg) was markedly elevated (16.6 mg/dl =13.7 mEq/l). Emergency colonoscopy revealed ischemic colitis. As her condition ameliorated, her renal function returned to normal. Hence, the present case suggests that severe hypermagnesemia can occur in the absence of pre-existing renal dysfunction in elderly patients with gastrointestinal diseases.