Functional failure of most organ systems of the body usually gives some indication of the organ at fault. If the heart is abnormal, there are murmurs and perhaps cyanosis; if the gastro-intestinal system is involved, there are vomiting, distention, abnormal stools, etc.; if the endocrine system is affected, there are alterations of body contours, changes in the skin and its appendages, and abnormalities of the secondary sexual characteristics. In addition, there is often an associated failure of growth. But, if the urinary system is defective, the first sign may be only failure to grow, with no reference to the source of the difficulty. The alert pediatrician, confronted with physical retardation in a child without obvious cause, will immediately direct his attention to the urinary tract and call upon his radiologic and urologic colleagues for assistance. The presence of an abdominal mass in infancy and childhood, particularly under the age of five years, involves the differential diagnosis of Wilms' embr...