Patients with angioedema are often referred to an allergist to rule out an allergic cause. In most of these cases, no allergic cause is identified, and the cases are labeled "idiopathic." Occasionally, a deficiency of the inhibitor of the first component of complement (C1INH) is discovered, which may be either hereditary or acquired. In comparison with the hereditary variant, the acquired deficiency of C1INH is extremely rare, approximately 40 cases having been reported to date. Measurement of the C1q subunit is the key to the differential diagnosis between hereditary and acquired deficiencies of C1INH--it is normal in the former and decreased in the latter. The acquired deficiency of C1INH is usually found in association with benign or malignant B-cell lymphoproliferative disorders, and the angioedema responds to therapy with androgens. A subset of six patients with acquired C1INH deficiency has been reported recently; they had anti-C1INH autoantibodies, no evidence of an underlying disease, a benign course, and variable responses to therapy. Two new cases of angioedema in patients with acquired C1INH deficiency are described in this report. One patient had no evidence of an underlying disease 11 years after the onset of angioedema. The other case was associated with a B-cell lymphoproliferative disorder that became evident 9 months after C1INH deficiency was diagnosed, and androgen therapy stopped the attacks of angioedema. In this second case, the functional activity of C1INH mirrored the clinical response to therapy.