Background: Hereditary angioedema (HAE) is still underdiagnosed or diagnosed after a serious delay. This study aimed to evaluate the diagnostic delay (DD) and misdiagnosis of HAE, and to explore the risk factors associated with a longer DD. Methods: An Internet-based survey was sent to 129 patients with type 1 and 2 HAE who presented to the Allergy Department, Peking Union Medical College Hospital between 1983 and 2017; 107 patients (82.9%) responded, among whom, a total of 96 patients provided complete information about medical visits. DD was divided into two subperiods according to the lower quartile, i.e., DD ≤ 6 years and DD > 6 years. Results: The median DD of all 96 patients with HAE was 11.04 years (interquartile range [IQR], 6.06-18.27 years). A significant difference (p < 0.001) in the median DD was found between different decades of onset, i.e., before 1999 (19.75 years [IQR, 13.58-29.50 years]), between 2000 and 2009 (8.67 years [IQR, 5.67-11.04 years]), and between 2010 and 2017 (3.79 years [IQR, 2.29-5.71 years]). Patients with a previous misdiagnosis experienced a longer median delay to complement 1 inhibitor HAE diagnosis (13.17 years [IQR, 7.40-20.50 years]) compared with patients without a previous misdiagnosis (median 6.96 years [IQR, 2.83-10.65 years]; p ≤ 0.001). According to the logistic regression analysis, a younger age of onset and earlier decade of onset were significant predictors of a DD of >6 years. The most frequently visited departments and most frequently misdiagnosed diseases were summarized. Conclusion: The median DD of patients with HAE was 11.04 years (IQR, 6.06-18.27 years). A younger age of onset and earlier decade of onset were predictors of a DD of >6 years. Seventy-five percent of the patients reported receiving more than one previous misdiagnosis. The patients with a previous misdiagnosis had longer DDs compared with patients without a misdiagnosis.