Congenital C1-inhibitor deficiency, or hereditary angioneurotic edema (HAE), is a rare autosomal dominant disease due to alterations in the C1 inhibitor gene that results in a deficiency of antigenic and/or functional C1-INH. Affected patients are heterozygous, and their deficiency is incomplete, many of them having up to 20% of the normal amount of the inhibitor. The disease is characterised by recurrent, circumscribed, non-pitting, and non-pruritic subepithelial swellings of sudden onset, which fade during the course of 48-72 hours, but can persist up to 1 week. Lesions can be solitary or multiple and primarily involve the extremities, larynx, face, and bowel wall. Bradykinin is believed to be the main, but certainly not the sole, mediator responsible for the bouts of edema in HAE. The diagnosis is suggested by family history, the lack of accompanying pruritus or urticaria, the presence of recurrent gastrointestinal attacks of colics, and episodes of laryngeal edema. Diminished C4 concentrations during symptomatic periods are highly suggestive for the diagnosis. Further laboratory diagnosis depends on demonstrating a deficiency of C1-INH antigen (type I) in most kindreds, but some kindreds have an antigenically intact but dysfunctional protein (type II) and require a functional assay to establish the diagnosis. Prophylactic administration of either attenuated androgens or protease inhibitors has proved useful in reducing frequency or severity of attacks. Infusions of a vapour-heated C1-INH concentrate are safe and effective means of both preventing and treating attacks. Nevertheless, this treatment is expensive and this extract is not readily available. It is emphasised that administration of angiotensin converting enzyme inhibitors is contraindicated in patients suffering from protease inhibitor deficiency states.