Acquired C1 esterase inhibitor deficiency is a clinical syndrome closely resembling hereditary angioedema in which most patients have an associated malignancy of B cell lineage. Sera from 33 patients with B cell neoplasms were assayed for C1 esterase inhibitor level by rocket immunoelectrophoresis. The clinical summary of a patient who met the biochemical criteria for acquired C1 esterase inhibitor deficiency (low C1 esterase inhibitor, low C4) is presented. We hypothesize that this syndrome represents an autoimmune reaction involving tumor cell rejections and of potential benefit to the patient.