The incidence of congenital complete heart block is estimated in 1 of 2500-20,000 births. Many cases are isolated (found in an otherwise normal heart) and the pathology of the heart conduction system is variable. We report a 51-year-old man with the diagnosis of complete heart block, with a permanent pacemaker. No family history of rhythm disturbances was available. The patient presented and endocarditis after replacement of the pacemaker battery. The prognosis was poor and the patient died three months later. Autopsy examination showed signs of shock, of septic origin. The heart was hypertrophic (450g) and the left ventricle thickened. Histopathological examination of the heart conduction system showed that the sinus and atrioventricular nodes were normal, but the His bundle was interrupted and replaced by fibrous tissue. No inflammatory signs were present. Loss of conducting fibres and their replacement by fibrous tissue is the most common pathological process in complete heart block. In this case His bundle was mostly affected, different to Lev's disease where the process is more distal (branching atrioventricular bundle) and to Lenegre's disease, which shows a diffuse damage in the conducting system.