A case of hereditary angioedema (HAE) type I (inherited C1-inhibitor [C1 INH] deficiency) and a case of late-onset acquired C1 INH with angioedema is described. In both patients, long-term prophylaxis with C1 INH had become necessary because treatment with danazol and epsilon-aminocaproic acid was not effective or not tolerated. Consequently, both patients received a pasteurized concentrate of C1 INH continuously for a period of 1 year in a dosage that kept them free of symptoms. The patient with HAE was administered 500 units of C1 INH intravenously every 4 or 5 days, whereas the patient with acquired angioedema required 1000 units of C1 INH every 5 days. As a result of this long-term prophylaxis, both patients became free or nearly free from their episodes of cutaneous and internal edema. The low plasma levels of C1 INH, C4, and C2, rose. In the patient with acquired C1 INH deficiency, the swellings increasingly reappeared after 10 months, although the patient's antibody titer did not rise during treatment. No side effects were recorded during therapy. In particular, both patients remained HIV and hepatitis B antibody negative.