This 3-part study will evaluate the efficacy and safety of an oral kallikrein inhibitor, BCX7353, in the treatment angioedema attacks in subjects with Type I or II hereditary angioedema (HAE). In each study part, subjects will treat 3 attacks with BCX7353 (2 attacks) or placebo (1 attack), in a randomly allocated order. In Part 1, the dose of 750mg will be assessed relative to placebo in up to 36 patients. If this is shown to be effective, then a further 12 patients will be enrolled at a 500mg dose (Part 1), followed by a further 12 (if efficacy still shown) at a dose of 250mg (Part 3) to determine the minimum effective dose of BCX7353 compared to placebo for treating HAE attacks. Efficacy will be determined by subject diary entries completed at pre-defined times post-dose.

Inclusion Criteria:

1. Able to provide written, informed consent.
2. A clinical diagnosis of hereditary angioedema Type 1 or Type 2 as documented at any time in the medical records or at the screening visit.
3. Access to and ability to use standard of care acute attack treatment for attacks of HAE.
4. Sexually active women of child-bearing potential and sexually active men must utilize effective contraception.

Exclusion Criteria:

1. Women who are pregnant or breast-feeding.
2. Any clinical condition or medical history that would interfere with the subject's safety or ability to participate in the study.
3. Use of C1INH, androgens or tranexamic acid for prophylaxis of HAE attacks.
4. History of or current alcohol or drug abuse.
5. Infection with hepatitis B, hepatitis C or HIV.
6. Participation in any other investigational drug study currently or within the last 30 days.
7. Positive drugs of abuse screen (unless as used as medical treatment, e.g., with a prescription).
8. An immediate family relationship to either Sponsor employees, the Investigator or employees of the study site.