Most patients experience their first attack during childhood or adolescence, and then continue to suffer from subsequent attacks throughout the duration of their lives.

An episode of HAE involves an acute swelling of the skin or mucous membranes, primarily affecting the limbs, face and neck as well as the buttocks and genitalia. Although this swelling of the skin is almost never associated with itching, it causes a feeling of tension, the severity of which ranges from unpleasant to painful. The facial swelling can be disfiguring. Swelling can also occur in the wall of the intestine. This swelling is associated with colicky pain and can cause diarrhea, vomiting and circulatory disorders. In extreme cases, even intestinal paralysis and intestinal obstruction are possible.

The most feared complication of HAE is swelling in the larynx (laryngeal edema) or upper respiratory tract, which can cause death by suffocation if left untreated.

The frequency, duration and severity of attacks vary considerably. Approximately 22 percent of patients report a frequency of greater than one attack per month; 40 percent of patients experience on average 6 to 11 attacks per year; and the remaining 22 percent are infrequently symptomatic.

In most cases, the symptoms of HAE are transient, progressing over 12 to 36 hours, and then subsiding gradually over the next two to five days. However, some patients may experience attacks that last over a week. Although HAE episodes are self-limiting, the unpredictable occurrence of attacks places considerable strain on patients, often restricting their ability to lead normal lives. Untreated, patients with HAE lose up to 100 to 150 work days per year and have morbidity approaching 50 percent.