Long term anticoagulant therapy was begun following the demonstration of a thrombosis of the right internal carotid artery. Agranulocytosis appeared abruptly during the 4th week of therapy resulting in death within 3 days. This drug is generally considered to be non-toxic, however 6 instances of agranulocytosis have previously been reported, one of which was fatal. In every instance agranulocytosis developed during the 4th or 5th week of therapy. The drug is preferred by those who use it principally because of its rapid onset of action. A method of initiating anticoagulant therapy designed to take advantage of this quality while at the same time obviating the danger of a possibly fatal reaction is described. It is suggested that there is little indication for the use of phenylindandione beyond the first week of therapy.