THE PURPOSE of this report is to describe the unusual circumstance of a patient with congenital factor IX or plasma thromboplastin component (PTC) deficiency (Christmas disease) who had clinical evidence of continued hypocoagulability throughout his lifetime but, nevertheless, spontaneously developed a myocardial infarction when he reached the sixth decade. In addition, a second and fatal coronary occlusion occurred 11 years later while being treated in the hospital for uncontrolled bleeding and at the point where plasma replacement had completely ameliorated the hemorrhage. These observations are of interest because they have bearing on the general concept of blood coagulability as it affects the genesis of occlusive coronary artery disease. Report of a Case A 67-year-old white man was admitted to the Graduate Hospital of the University of Pennsylvania in November 1962 because of oral bleeding of three days' duration. The blood loss, estimated to be one pint, occurred in the