BACKGROUND: The main indications for implanting a cardiac pacemaker are either acquired atrio-ventricular block (AVB) or sick sinus syndrome (SSS). In general, the exact aetiology of these conditions is unknown. Genetic factors may play a role. Untreated AVB is associated with increased mortality but SSS seems to have a good prognosis. The present study was designed to assess whether first-degree relatives of these patients have an increased mortality risk. METHODS: We investigated the survival pattern of the co-twins of twins who have had a cardiac pacemaker implanted, by linking two nationwide registries in Denmark: the Danish Pacemaker Register and the Danish Twin Registry. For each "pacemaker-twin" four control-twins matched with respect to age, sex, and zygosity were selected among the twins who were alive at the time of the pacemaker implantation. A total of 159 twins with a pacemaker treated bradycardia of unknown aetiology were identified of which 73 had AVB and 64 had SSS. A total of 622 matched control-twins could be identified. The survival from age six of the co-twins of the "pacemaker-twins" was compared with that of the co-twins of the control-twins. The follow-up period ended January 1st, 2000. FINDINGS: The survival curves for the co-twins of the "pacemaker-twins" were virtually identical to those of the co-twins of the control-twins. This finding was consistent for both AVB and SSS and for monozygotic as well as dizygotic twins. CONCLUSION: No increased mortality risk could be demonstrated for first-degree relatives--including monozygotic twins--of patients with a cardiac pacemaker implanted due to either acquired AVB or SSS.