Despite the rapid advance in our knowledge of cardiovascular disease, pathologic examination of the heart frequently fails to disclose the conditions which are anticipated on the basis of the clinical diagnosis. The clinical diagnosis of "coronary thrombosis" is often made but no thrombosis may be found; in other instances a fresh infarct is disclosed but no fresh thrombosis of any arteries supplying the affected area may be evident. Conversely, fresh coronary thrombosis or complete occlusions of main coronary arteries may be found post mortem but without any corresponding clinical phenomena. Because of the frequent disparities between clinical diagnoses and pathologic observations a detailed clinical and pathologic study was made in a consecutive series of necropsies. In the pathologic examination of the heart, a method devised by one of us (M. J. S.) was used,1since it affords more precise information regarding the structural changes of the coronary arteries. The