This study will enroll people admitted to the emergency room or hospital who have a severe case of ACE inhibitor-associated angioedema. Participants will be randomly assigned to receive an injection of either HOE-140 or placebo. Initially, participants will undergo an electrocardiogram to measure the electrical activity of the heart. Then blood pressure measurements, blood collection, a physical exam to determine the extent and duration of swelling, and photographs of the swelling will occur at 2, 4, 8, 16, and 24 hours following the start of treatment. Questionnaires will be completed by study staff and participants to assess changes in angioedema symptoms and the extent of swelling. Participants will remain in the hospital for 24 to 48 hours, depending on the severity of their symptoms. Blood will be collected at a follow-up visit that will occur 7 days after the resolution of angioedema symptoms.