We have studied the incidence of thromboembolic complications in 46 patients. Twenty-one patients had a Medtronic-Hall prosthesis implanted in the mitral position (group A) and 25 patients had the same prosthesis implanted in the aortic position (group B). Several factors were defined initially as "thromboembolic risk factors" and we have tried to establish a correlation between those factors and the actual thromboembolic complications. Group A: 21 patients presented 5 thromboembolic accidents (23.8%). Preeminent factors were: preoperative thromboembolic phenomena, previous heart operations, atrial fibrillation and the presence of congestive heart failure and/or low outpout syndrome in the postoperative period. Group B: 25 patients presented 6 thromboembolic accidents (24%). If we exclude the presence of low outpout syndrome in the postoperative period, there was no correlation between other risk factors and the incidence of thromboembolic complications. We must mention that the incidence of thromboembolic complications in this group diminished significantly (26% to 16%) when oral anticoagulation was continued with aspirin. CONCLUSIONS these figures of thromboembolic complications appear too high at first sight if we consider only their absolute value, but are justified when one bears in mind the clinical circumstances of the patients who suffered from thromboembolic complications.