Eighty-five male patients with acute myocardial infarction, pulmonary infraction, congestive heart failure, phlebitis, and cerebral and peripheral emboli were treated with warfarin to observe the clinical effects and the results of long-term therapy. The effect of warfarin was evaluated in terms of both "optimal" and "suboptimal" prothrombin depression (2 to 2 1/2 times and 1 1/2 to 2 times of control respectively). Warfarin is the only available anticoagulant which is clinically administered in terms of milligrams per kilogram of body weight. Administration of 0.75 to 1 mg. per kilogram induces a prompt therapeutic hypothrombinemia in the vast majority of patients. The maintenance effects of warfarin, observed in half of the patients for more than four weeks, are relatively predictable, consistent, and safe.