A complete atrioventricular block (CAVB) can be a lethal complication when it is not treated directly with isoprenaline and pacemaker therapy. The overall incidence of CAVB varies between 4 to 8 per cent with a mortality OR of 3.2 within 30 days if untreated. Main causes of CAVB are inferior myocardial infarction, congenital AV node malformation, mitral valve insufficiency and valve surgery, metabolic disorders and intoxications. The authors describe a case with a CAVB due to lithium-clozapine therapy and relapsing multiple sclerosis.