The long-term effects of anticoagulant therapy have been studied in 100 patients with evidence of cerebral vascular thrombosis or embolism. During 2,842 patient-months without anticoagulant therapy there were 229 thromboembolic episodes (67 being cerebral) as compared with 20 thromboembolic episodes (5 being cerebral) that occurred during 2,291 patient-months on anticoagulant therapy. Similar data permitted comparison of continuous with interrupted anticoagulant therapy, and it was found that 48 thromboembolic episodes occurred during 1,311 patient-months on interrupted therapy as compared with 15 episodes during 957 patient-months on continuous therapy. The dosages of anticoagulant (usually bishydroxycoumarin) were aimed to keep the prothrombin time for the undiluted plasma between 20 and 40 seconds. Hemorrhagic complications occurred in 30 of the 100 patients; there were three fatal cerebral hemorrhages, occurring over a period of 2,532 patient-months of anticoagulant therapy. The results are interpreted to mean that anticoagulant therapy is effective in reducing the danger of recurrent thromboembolic episodes if contraindications are observed and prothrombin times are pains-takingly controlled.