Anticoagulation therapy with acenocoumarin or with anticoagulants plus aspirin was given to 65 and 57 patients, respectively, with cardiac valve replacement. The follow-up was 1,462 months (22.5 months per patient) for the first group and 1,411 months (24.7 months per patient) for the second group. The frequency of embolic accidents was significantly lower in the group taking aspirin: Thirteen thromboembolic accidents were detected in patients receiving the anticoagulant and 3 in the group receiving the anticoagulant plus aspirin. These figures represent a 20.3 per cent incidence (one each 9.3 years of treatment) for the anticoagulant group and a 5.2 per cent incidence (one accident each 39.1 years of treatment) for the other group. The statistical significance between groups is p less than 0.005. There was no difference in the hemorrhagic risk between the two groups. We conclude that the use of an anticoagulant plus aspirin is a good and safe therapy for the prevention of thromboembolism in these patients.