The association of late onset recurrent angioedema with a deficiency of the inhibitor of the first component of complement (C1INH) and of the binding subunit of the first component, Clq, defines the syndrome of acquired C1INH deficiency. The description of five new cases, along with the original two and the 18 others in the literature, brings the total reported cases to 25 and highlights the associated B cell abnormalities that are present in 23 and are of a malignant nature in 19 cases. In three of the five newly reported cases, the occurrence of angioedema, which prompted recognition of the acquired deficiency of C1INH, C1q, and C4, preceded the delineation of the underlying B cell malignancy by 2 to 3 yr despite efforts to recognize neoplastic disease in two of these patients throughout the interval. Because the acquired C1INH deficiency reflects increased catabolism rather than impaired biosynthesis, only high-dose attenuated androgens elicit a measurable increment in serum C1INH. The occurrence of the syndrome with multiple myeloma is noted for the first time.