Helicobacter pylori infection is thought to be a causal factor in various dermatological disorders. We assessed the frequency of H pylori infection in 65 patients with hereditary angioneurotic oedema. We measured the serum concentration of antibodies against H pylori and did the carbon-14-urease breath test in patients with positive H pylori serology. 19 of 65 patients had H pylori infection. All patients with infection, and 11 of 46 without infection, had a history of recurrent episodes of acute abdominal pain. We successfully eradicated H. pylori infection in 18 patients. The frequency of abdominal symptoms was significantly higher in the infected group (p=0.002 after adjustment for age). In nine of 19 patients with dyspepsia, the frequency of oedematous episodes decreased from 100 over 10 months before eradication to 19 during the 10-month follow-up period. Screening for, and eradication of, H pylori infection seems to be justified in patients with hereditary angioneurotic oedema.