Sarcoma, primitive or secondary, of the kidney is a somewhat rare occurrence, but most frequent in children. The whole kidney may be transformed into a mass occupying its place and somewhat resembling it in form, but many times exceeding it in bulk and weight. Such a tumor may largely distend the abdominal cavity and compress its contents. Upon section we often find a substance varying greatly in consistence, from almost fibrous hardness to cavities filled with grumous material broken down by fatty degeneration and often colored by hemorrhage. In the interior may be found remains of the pyramids and cortical substance occupying their usual relative positions, but as it were distended, these portions being surrounded by a much thicker layer of purely abnormal neoplasm, probably connected with the capsule and its surrounding fat. In other cases all traces of normal form and structure may have disappeared. The microscopic structure of such a growth presents no peculiarity except so far as the arrangement of cells in the normal gland may be followed to a certain extent in the less-altered portions of the tumor. Besides this total destruction of the kidney, it is not uncommon to find nodules involving a part of one or both the organs, and more or less distinctly marked off from the healthy portion. The origin of sarcomata involving the kidneys may be the subperitoneal cellular tissue or the neighboring organs. As a primary disease sarcoma of the kidneys is very rare. True cancer or carcinoma of the kidney is not a common disease, and is said to have been found 12 times in 447 cases of cancer of various organs. It may be primary or secondary, and a description of the gross appearances would be essentially the same as that of the sarcoma. The tumor does not, however, usually attain so large a size, and the amount of degeneration of neighboring organs and of ulceration is greater. Calculi are often found in cancerous kidneys.