Coumadin sodium and bishydroxycoumarin were used in a study of the effects of anticoagulant therapy on 200 patients, most of them with acute myocardial infarction or severe coronary insufficiency. Coumadin sodium was given orally to 164 of these patients, the prothrombin times being determined daily. Individual dosages were adjusted until the daily dose (usually 5 to 7.5 mg.) needed to maintain the prothrombin level between 15 % and 30% was found. Bishydroxycoumarin was given to 19 patients and dosages were similarly adjusted until the maintenance dose (between 20 and 100 mg. daily) was found. An additional group of 17 patients who at first received bishydroxycoumarin were shifted to therapy with coumadin sodium. Coumadin sodium was found to be far superior to bishydroxycoumarin. Coumadin sodium had the advantage of greater promptness of effect; it produced the desired prothrombin levels within 48 hours in 87% of the cases. Patients receiving it in maintenance doses were more likely to stay within the desired range of prothrombin levels. The frequency of side-effects (mainly hemorrhagic) was slightly though not significantly lower in the patients treated with coumadin sodium. Nine patients who exhibited bleeding phenomena promptly responded to withdrawal of the coumadin sodium or treatment with preparations of vitamin K.