Hereditary angioedema (HAE) is a rare autosomal-dominant disease characterized by recurring attacks of nonpruritic, nonpitting edema caused by an inherited deficiency or dysfunction of C1 esterase inhibitor (C1 INH). Symptoms can present years before an accurate diagnosis is made. The objective of this study, the Angioedema Clinical Epidemiology Testing Initiative for the Study of Hereditary Angioedema, was to determine the prevalence and clinical manifestations of HAE in untested first-degree blood relatives of known patients with HAE. Patients with a confirmed diagnosis of HAE recruited first-degree relatives who had not been evaluated for HAE. Enrolled subjects underwent complement testing (C4, C1 INH antigen, and functional C1 INH). If the lab tests were abnormal, the enrolled subjects returned to the site for a follow-up visit and questionnaire. Overall, 31 patients with HAE and 46 first-degree relatives enrolled in the study. Of 46 enrolled relatives, 30 (65%) had lab test results that ruled out a diagnosis of HAE, two (4%) were categorized as "HAE not ruled out," and 14 (30%) were newly diagnosed with HAE. Of 14 newly diagnosed subjects, nine (64%) reported having experienced symptoms that may have been related to HAE, such as swelling in the throat, face, or extremities or abdominal pain. When reported, median age of symptom onset in these 14 subjects was nine years whereas newly diagnosed asymptomatic subjects had a median chronological age of six years. These 14 subjects reported a historic mean standard deviation rate of 2.51 (5.59) swelling episodes per month with a mean standard deviation duration of 1.6 (0.74) days. This study's findings reinforce the importance of testing family members of patients with HAE to detect this hereditary condition.