Angio-oedema is a rare but potentially life threatening side-effect of angiotensin-converting-enzyme (ACE) inhibitor treatment. Identification of individuals at risk of this adverse effect is not possible. Angio-oedema is associated with raised concentrations of bradykinin, which is mainly inactivated by ACE. We assessed the plasma activity of two other enzymes that catabolise bradykinin (aminopeptidase P and carboxypeptidase N) in 39 hypertensive patients with a history of angio-oedema during ACE inhibitor treatment and in 39 hypertensive patients who had never had ACE inhibitor associated side-effects. Patients with previous angio-oedema had a lower plasma activity of aminopeptidase P than did those who never presented with angio-oedema (p=0 003). Our data suggest that low plasma concentrations of aminopeptidase P could be a predisposing factor for development of angio-oedema in patients treated with ACE inhibitors.