The investigators will conduct a double-blind, randomized controlled trial comparing the safety and effectiveness of ecallantide to conventional therapy. A rescue cross-over design will be used such that patients failing to improve on standard therapy will additionally be treated with ecallantide. Therefore, a historical control cohort will be enrolled for analysis of secondary endpoints. In addition, since some patients treated with conventional therapy may improve rapidly and therefore not be eligible for inclusion in the study, the investigators will enroll these patients as an observational arm to enable the conduct of sensitivity analysis.

Inclusion Criteria:

1. Males and females of any race or ethnicity 18 or older. The locations of the study will permit all racial and ethnic distribution in the study
2. Must currently be on an ACE inhibitor
3. Presenting with ACE induced angioedema within 12 hours after onset. This is to be documented in the source document and CRF
4. All females of childbearing age must have a negative pregnancy test prior to administration of the study drug.

Exclusion Criteria:

1. Participation in another investigational study within 30 days prior to enrollment
2. Patients who improve on conventional (standard of care) therapy
3. Patients previously treated with ecallantide
4. Hypersensitivity to ecallantide
5. Pregnancy or breast feeding
6. Other definable causes of angioedema (i.e., hereditary or acquired angioedema)
7. Patients receiving C-1 inhibitor as prophylaxis
8. Treatment requiring tranexamic acid, and epsilon-aminocaproic acid
9. Receiving fresh frozen plasma within 3 days prior to enrollment