For ages signs and portents have been an important factor in the life of many human beings. Faith in signs of different kinds may rest largely or entirely on superstition or may be based on a substratum of solid facts. Among the latter must be classed certain effects of disease or, in other words, certain functional or organic changes associated with and therefore characterizing certain diseases, and by the recognition of which one kind of disease may be distinguished from other kinds. Diagnostic roentgenologists are prone to believe in signs, roentgenologic signs, of course, because experience has taught them that certain signs or appearances are always or usually associated with this or that disease. When certain signs have been widely accepted by a large number of experienced physicians as reliable and constant indications of any pathologic disturbance, they pass from the realm of faith into that of knowledge or fact. Many roentgenologic appearances have acquired the value of pathognomonic signs, provided they are recognized and properly interpreted. Thus it is that the knowledge and experience of the observer play such an important part in roentgen interpretation. Besides those well established and near classical signs of certain diseases, there are other signs which, for a time, may be thought to represent a certain condition, but on extended experience may prove to be relative rather than absolute, or to indicate a variety of tissue reaction induced by a number of analogous causes rather than a specific and invariable reaction to a single cause. When, in 1921, Ewing first described the solitary diffuse endothelioma of bone to which his name has since been so commonly attached, he was led to recognize it as a well defined and specific neoplasm by four main features: its peculiar histopathologic character, relating it to the endothelium of blood vessels or lymph channels in bone marrow or bone; the roentgenographic appearance of diffuse rarefaction of a large portion of the widened shaft of the affected bone; the absence of bone production; its exceptional sensitiveness to roentgen rays or radium. These features, together with the peculiar history which characterizes such tumors, are sufficient to distinguish them from other malignant tumors of bone. And yet, even now, many cases are not recognized, but are classed as cases of round-cell or osteogenic sarcoma, and, at the outset, some are mistaken for cases of tuberculosis.