Symptomatic hypermagnesemia is rare and can be induced by exogenous magnesium-containing cathartics or antacids. We report a patient with hypermagnesemia. The patient was treated with continuous hemodiafiltration (CHDF); however, he died on the 4th hospital day. Hypermagnesemia is not easily detected because the magnesium level is not examined during routine investigations, and many physicians are relatively unfamiliar with hypermagnesemia. Hypermagnesemia should be considered in elderly patients presenting with hypotension, bradycardia, hyporeflexia, or respiratory depression, and particularly in patients with abnormal renal function or small bowel hypomotility. Magnesium-containing cathartics or antacids should be used more carefully in the elderly.