Hereditary angioedema is a rare disease resulting from a lack of functional C1 esterase inhibitor (C1 INH). Several genetic defects can cause decreased production of the protein or the synthesis of a biologically inactive form. A similar, acquired condition is occasionally seen, associated with malignancies or as an autoimmune process. Disease severity varies greatly among affected individuals. Most patients have cutaneous, laryngeal, or gastrointestinal edema, often in combinations. The symptoms may appear spontaneously or result from a stimulus, usually trauma. When clinical suspicion exists, measurement of the C4 level screens for the disease. An assay showing low serum C1 INH function confirms the diagnosis. When disease severity warrants, symptoms can be controlled with anabolic steroids or antifibrinolytics. Doses should be increased before symptom-provoking events. Emergencies are treated with plasma infusions, fluids, and pain control. Where available, C1 INH concentrate is the treatment of choice. Therapy can usually be monitored by control of symptoms. With appropriate therapy, most cases remain well controlled.