The complement system plays an important role in defence of the host against infection and in the clearance of immune complexes. Defects in complement proteins are often associated with infections or auto-immune/immune complex diseases. Investigation of complement is useful for diagnosis and following of autoimmune diseases. Hypocomplementemia may result from genetic deficiency and/or consumption through activation of the system. In the particular case of angioneurotic oedema, laboratory data are essential to confirm the diagnosis and to the follow-up of the patients. Inappropriate activation of the complement system associated with the release of inflammatory mediators contributes to the tissue damage.