NINE years ago, it was my privilege to speak before this Society on the relation of radiology to urology and to welcome the partnership existing between our two branches of medicine. To-day we find this bond even closer. This close affiliation is particularly essential in the radiologic diagnosis of renal tumors and polycystic kidneys. It is manifest that either urology or radiology alone is inadequate to explain the diagnostic intricacies of renal tumors, and even their combined efforts occasionally fall short, the aid of other general and special studies being sought. Many of these, however, are dependent upon the x-ray and demand complete gastro-intestinal configuration and cholecystography, as well as general skeletal studies and roentgen-ray examination of the chest for metastatic changes. Study of the genital tract in male or female may disclose the primary cause of some vague abdominal or retroperitoneal tumor. Provided all renal neoplasms uniformly presented the symptom-complex of tumor, pain, and hematuria, the task of diagnosis would be simplified, but only one-third of the cases record all three, hence the need for detailed study. My listeners familiarity with this subject makes it extremely difficult for me to offer any novel features; hence in this presentation I can do no more than refresh your memory with some of the standard facts in diagnosis and possibly suggest a few points which may elicit discussion. Thoroughly to differentiate renal tumors, it is necessary to consider not only new-growths of the kidney but other pathologic processes such as severe infections (with or without calculus), tuberculosis, parasitic diseases, large hydronephroses, abnormal movability, and congenital malformations which may bear a close resemblance to neoplasms. The larger growths often require accurate differentiating from gall-bladder and splenic conditions, intestinal tumors and retroperitoneal masses. Again, we must be equally alert to the changes in the pelvic outline in cases presenting symptoms suggestive of tumor but without visible or palpable evidence of the growth, such as those occurring from small neoplasms invading a localized area of the kidney substance and pelvis, tumors within the pelvis, and filling defects produced by less serious causes such as those created by spasm, soft stones, aberrant vessels, granulomas, and vascular changes. Both groups offer many complex problems. Since we are dealing with the value of radiology in the diagnosis of such tumors, other diagnostic aids will receive no comment, assuming, of course, that a thorough medical investigation has been made. The size, contour, presentation, location, and the relation to other viscera, particularly the bowel, are helpful aids, but ultimately the combined efforts of urologists and radiologists are necessary for the differentiation of abdominal tumors.