THE renaissance in urology followed closely upon three great discoveries—that of the cystoscope, that of the renal function test, and, finally, that of the roentgen ray. With the judicious application of these several aids, urology, at least in so far as its diagnosis is concerned, has developed to a degree hardly approached by that of any other special branch of medicine. A very large share, if not actually most, of this progress in the diagnosis of diseases of the kidney, ureter and bladder has been made possible through the advent of the X-ray. The plain roentgenogram alone has taken the entire subject of urinary stones out of the field of rank speculation and classified it with the near-exact sciences. A very certain, though much overrated, value has come to be attached to the diagnostic significance of the renal outline as revealed in the plain roentgenogram. Of far greater moment than either of these, the visualization of the outline of the cavity of the urinary tract by means of the cysto-, uretero-, and pyelogram has come to be the “open sesame” of urological diagnosis. Like most great blessings, however, that of the roentgenogram in urological diagnosis has come to us not entirely without potentialities for grave danger. There is nothing farther from my mind than denying or belittling to the slightest degree the immense worth of the roentgenogram to the urologist. Just to the contrary, I am deeply interested in stressing the various points that tend to enhance the value of the X-rays to this specialty. If permitted a certain latitude with the subject assigned me, I should like to take up the matter of urinary lithiasis, with special reference to its roentgenological aspects. The consistency with which stones of the urinary tract cast clear and definite shadows on the X-ray film at once makes such a means of investigation absolutely para mount in the diagnosis of this condition. In fact, the greatest danger of error in the diagnosis of urinary stones lies in the very ease and simplicity of radiography. That may seem somewhat of a paradox until one stops to consider that not all shadows suspected of being stone shadows are caused by urinary stones, and that, after all, urinary stones are not always a disease in and of themselves but are frequently found occurring simply as a complication of more fundamental urinary pathology. If bitter experience has been the teacher of any one outstanding lesson in diagnosis, that lesson has been “never to cut corners.” The diagnostician, whether he be urologist, radiologist or what not, who depends solely upon the plain roentgenogram in the investigation of urinary stones not only does so at the risk of occasional grave error but in so doing fails miserably in an appreciation of the real worth of the roentgen ray in urology. The danger in the diagnosis of urinary stones by the plain plate lies in two things.