Experience in the first seven years of a double-blindfold controlled study of continuous prophylactic anticoagulant therapy following myocardial infarction has demonstrated no difference in survival of patients receiving phenindione as compared with those receiving placebo treatment or no treatment. All patients received anticoagulant therapy for six weeks during initial or recurrent acute myocardial infarctions. All were selected for study before being assigned randomly to treatment groups. Longer hospitalization was required for anticoagulant-treated patients. There was no appreciable reduction of arterial thrombo-embolic complications.