Right atrial thrombosis is a rare complication of permanent endocardial pacing. We report two cases with large right atrial thrombi which occurred during permanent endocardial electrical stimulation. Both patients were women, aged 83 and 89 years, who died suddenly, respectively, one month and 24 days after pacemaker implantation. At the time of death the two patients were in severe persistent refractory congestive heart failure despite appropriate medical treatment. At necropsy large mobile right atrial thrombi were found in both cases.