The blood urea has been examined in 106 cases which include cases of appendicitis, saplingitis, cholecystitis, puerperal sepsis, various abscesses, peritonitis, intestinal obstruction, pneumonia, empyema and pleural effusion. It was also examined in four cases of myelaemia. Urea concentration tests were performed to exclude nephritis. The blood urea is raised in some cases of all of these conditions except cholecystitis and uncomplicated salpingitis. It is markedly raised in intestinal obstruction, in pneumonia before the crisis, and in general peritonitis when there is a paralytic ileus. The rise in blood urea, however great it may be, is always associated with perfectly normal urea concentration tests, and therefore it is not due to a failure of the kidneys to concentrate urea. An examination of the blood urea may have considerable diagnostic importance in abdominal cases, and a reading over 100 mgrs. per 100 cc. is strongly in favour of intestinal obstruction. A urea concentration test should always be performed as well as estimating the blood urea, especially when a question of uraemia is involved. The blood urea is no guide as to whether an abscess or empyema has developed or not.