Five elderly persons with senile dementia accidentally ingested Hoesmin, a 10% aqueous solution of benzalkonium chloride (BAC). The condition of one patient, an 84-year-old woman whose lips and oral cavity became erythematous, gradually deteriorated. Although gastric lavage was performed, the patient died 3 h after ingestion of Hoesmin. Autopsy revealed corrosive changes of the mucosal surfaces of the tongue, pharynx, larynx, esophagus and stomach which may have come in contact with BAC. In addition, BAC was detected in the serum. We conclude that the patient died of BAC poisoning. Fatal BAC poisoning is rare and autopsy findings in only a few cases of BAC poisoning have been reported. Our findings emphasize the risk of oral ingestion of BAC.